Water Control Plants Research Articles

First Report of Fusarium oxysporum f. sp. cubense Tropical Race 4 Causing Fusarium Wilt in Cavendish Bananas in Peru.

Fusarium oxysporum f. sp. cubense Tropical Race 4 (Foc TR4), the causal agent of Fusarium wilt of banana (FWB), is currently the major threat to the banana industry worldwide (Dita et al. 2018). Restricted to South Asia for more than 20 years, Foc TR4 has spread in the last years to the Middle East, Mozambique, and Colombia (García-Bastidas et al. 2019; https://pestdisplace.org/embed/news/map/disease/11). The incursion of Foc TR4 in Colombia increased awareness and prevention efforts across Latin American and the Caribbean (LAC). However, new Foc TR4 outbreaks in LAC countries were considered a matter of time. In April 2021, banana (Musa spp., Cavendish, AAA) plants (30% of incidence) showing typical symptoms of FWB, such as leave yellowing, wilting and vascular discoloration were observed in one farm (about 1 ha) located in Querecotillo, Peru (4°43'54.84"S 80°33'45.00"W). Mycological analyses of samples (pseudostem strands) collected from 10 symptomatic plants were performed as described by Dita et al. (2010). These analyses revealed a continuous presence of fungal colonies identified as Fusarium oxysporum species complex. Molecular diagnostics targeting two different genome regions (Dita et al 2010; Li et al. 2013) identified nine of these isolates as Foc TR4. These results were further confirmed by qPCR analyses using the commercial Clear Detections TR4 kit. The genome of four single-spore isolates (PerS1, PerS2, PerS3 and PerS4) was sequenced using the Illumina platform (MiSeq Kit, 2x151 bp Paired-End). The strain PerS4 was also sequenced using Oxford Nanopore (FLOW-MIN111; R10.3 chemistry) as described by Lopez-Alvarez et al., (2020). The generated draft assembly yielded 533 contigs for a size of 47 Mbp (BioProject: PRJNA755905), which is comparable with sizes of previously reported Foc TR4 strains (Asai et al. 2019; García-Bastidas et al. 2019; Maymon et al. 2020; Warmington et al. 2019; Zheng et al. 2018). The sequence assembly showed high contiguity (94.9%) and high similarity (95.48%) with the high-quality genome sequence of the Foc TR4 isolate 'UK0001' (Warmington et al. 2019). Further analyses to identify the presence/absence of full sequences for the putative effector genes (Secreted In Xylem - SIX) and their allelic copies, also revealed that the SIX genes profile of the strains isolated from Querecotillo matched with previously reported Foc TR4 isolates (Czislowski et al. 2017). Pathogenicity tests with three isolates and water controls were performed as described by Dita et al. (2010), using five Cavendish plantlets per treatment. Four weeks after the inoculation typical external and internal symptoms of FWB were observed only in the inoculated plants. Fungal isolates recovered from inoculated plants tested positive for Foc TR4 when analyzed with PCR diagnostics as mentioned above. No fungal isolates were recovered from water-control plants which did not show any symptoms. Altogether, our results confirm the first incursion of Foc TR4 in Peru. Currently, Foc TR4 has the phytosanitary status of a present pest with restricted distribution in Peru and it is under official control of the National Plant Protection Organization - SENASA. Reinforced prevention and quarantine measures, disease monitoring and capacity building to detect, contain and manage eventual new outbreaks of Foc TR4 are strongly encouraged across LAC banana producing countries, especially for those bordering Peru with larger banana plantations, such as Ecuador and Brazil.

Spatial allocation effects within a potentization basic research model – evidence for field-like effects of homeopathic preparations?

Background: The mode of action of ultramolecular homeopathic preparations is still unknown. Interactions between objects or entities can be grouped in four main general scientific categories: material, force-/field-like, entanglement-like or informational. Should homeopathic preparations have a field-like mode of action, there is greater probability of cross-contamination as long as the means to “shield” objects from each other is unknown. A field-like interaction would also lead to treatment at a distance effects that are distance-dependent. Aims: We analysed a set of experiments with Arsenicum album 45x treated wheat seedlings regarding a possible distance-dependent cross-contamination. Materials and Methods: We performed an a posteriori analysis of a set of 17 independent experiments [1,2] with wheat seedlings pre-treated with 1‰ arsenic. Three treatments were applied (Arsenicum album 45x, water 45x, or unpotentized water) with 150 seedlings in each treatment group per experiment. Seedlings were arranged in hanging plastic bags side-by-side in identically treated blocks of 10 seedlings. The 3x15 blocks were coded and randomly allocated to the three treatments. Wheat shoot length was measured after 7 days. Treatment effects were analysed as function of the position (1–10) within the blocks of 10 seedlings. Results: Analyzing all data, Arsenicum album 45x exerted an inhibiting effect (–3.2%, p=0.01) compared to both water and water 45x. When restricting the analysis to the outermost seedlings of all subgroups (pos. 1, 10), the treatment effect vanished (0.3%, p=0.92). In contrast, the innermost seedlings of all subgroups (pos. 5, 6) showed a treatment effect of –5.6% (p=0.02). Intermediate pairs of positions (pos. 2–4, 7–9) showed intermediate effects. Regarding shoot length, dependency on spatial position was observed for the plants of the water control groups, but not for the plants treated with Arsenicum album 45x. Conclusions: Whilst the effect of Arsenicum album 45x on wheat-shoot growth was not dependent on the spatial position within the subgroup, the water-control plants became smaller the closer they were to Arsenicum album 45x-treated seedlings. This observation is compatible with the existence of a field-like effect of homeopathic dilutions. Another possible explanation that cannot be ruled out by the present experiments, is contamination through the gas-phase. Future investigations of ultramolecular homeopathic preparations should control any such effects since they may mask treatment effects, leading to false-negative results. Closer investigation of the nature of this distance-dependent effect might contribute to identification of the mode of action of ultramolecular homeopathic preparations.

Effects of Prohydrojasmon on the Number of Infesting Herbivores and Biomass of Field-Grown Japanese Radish Plants.

Prohydrojasmon (PDJ), an analog of jasmonic acid (JA), was found to induce direct and indirect defenses against herbivores in non-infested plants. To test whether PDJ can be used for pest control in crop production, we conducted experiments in pesticide-free Japanese radish fields from October 4 to December 12 in 2015. Twenty-four Japanese radish plants in three plots were treated with a 100 times-diluted commercial formulation (5%) of PDJ (treated plants), and 24 plants in three different plots were treated with water (control plants) until November 29 every week. Throughout the observation period, the number of aphids, leaf-mining fly larvae, vegetable weevils, and thrips was significantly lower on the treated plants than on the control plants. In contrast, the number of lepidopteran larvae was not significantly different between the treated and control plants throughout the study period. Parasitized aphids (mummies) were also observed in both plots. Poisson regression analyses showed that a significantly higher number of mummies was recorded on the treated plants as compared to that on the control plants when the number of aphids increased. This suggested that PDJ application to Japanese radish plants attracted more parasitoid wasps on the treated plants than on the control plants. We also identified eight terpenoids and methyl salicylate as the PDJ-induced plant volatiles in the headspace of the treated plants. Some of these volatiles might be responsible for attracting aphid-parasitoid wasps in the field. However, for other insect pests, we did not find any natural enemies. Interestingly, the genes of the JA and salicylic acid signaling pathways were differentially upregulated in the treated plants. We also observed that the PDJ treatments induced the expression of the genes related to glucosinolate biosynthesis and the subsequent isothiocyanate formation. Additionally, the weights of both the aboveground and belowground parts of the treated plants were significantly lower than those of the respective parts of the control plants. These results indicated that the treatment of Japanese radish plants with a 100 times-diluted commercial formulation of PDJ induced their direct and indirect defenses against several insect pest species to reduce their numbers, and negatively affected their biomass.

Effects of sunn hemp (Crotalaria juncea) and Mexican sunflower (Tithonia diversifolia) leaf extracts on the development of Meloidogyne incognita on African indigenous vegetables

The effects of sunn hemp and Mexican sunflower extracts were evaluated on the development of Meloidogyne incognita on two vegetables, Amaranthus cruentus and Solanum macrocarpon in the screenhouse in 2017. Seedlings of each vegetable were inoculated with 3000 eggs of M. incognita per pot. There were five treatments; sunn hemp and Mexican sunflower extracts at 20 and 40 g leaf tissues/L water for each and a non-treated control. The treatments were laid out in a randomized complete block design in four replications. Data were collected on nematode counts, egg masses, developmental stages and gall-index. Extracts of sunn hemp and Mexican sunflower at 40 g leaf tissues/L water, significantly reduce nematode number, egg mass and gall-index in comparison with plants treated at 20 g leaf tissue/L water and non-treated control plants. Plants treated at lower rate were not different from the non-treated control plants in all parameters measured. The results showed that sunn hemp or Mexican sunflower at 40 g leaf tissue/L could be used in managing nematode pest.

Control of late blight in organic farming with low copper dosages or natural products as alternatives to copper

The aim of this paper is to explore appropriate alternative products to copper for the control of late blight disease in organic farming. In fact, due to its persistence and toxicity, copper appears incompatible with organic farming’s objectives of environmentally friendly farming. The effect of plant extracts, inorganic products, microorganisms and chitosan were investigated in vitro and in a greenhouse pot trial. The in vitro tests consisted of evaluating both the development of oomycete colonies on an agar medium supplemented with the tested products at various concentrations and the inhibiting effect of products on sporangial germination. The greenhouse trial was carried out on 5-week-old tomato seedlings cv. Cuore di Bue di Albenga. Tomato seedlings were transplanted into pots containing pasteurized soil and were then treated with the investigated products. After 24 h, they were inoculated with a sporangial suspension of Phytophthora infestans. The disease severity for each pot was estimated. The results showed a remarkable inhibitory effect on hyphae growth and sporangia germination of the tested products. Under greenhouse experiments, all treatments significantly reduced late blight in comparison with water control plants. The best results were obtained with the formulation containing copper at low doses and leaf licorice extract. In conclusion, the results of this study show that it is possible to reduce the dependency on copper in crop protection by using low copper dosages or natural products.

Yield Potential of Small Millets under Drought Condition

An experiment was conducted to study the yield traits under reproductive stage drought in selected small millets viz., finger millet (CO 15), little millet (CO 4) and barnyard millet (CO 2). The crops were imposed with drought during their flowering stage by withholding irrigation till the soil moisture reaches below 20 per cent. Barnyard millet performed better under drought at reproductive stage over other small millets, such as finger millet and little millet, in terms of more number productive tillers (11.16 plant-1), ear heads (12.99 plant-1), lesser reduction in ear head length (13.35 %), ear head weight (21.66 %) and 1000 grain weight (14.93 %) compared to well watered control plants. Little millet recorded the highest reduction in ear head length (19.01 %), ear head weight (39.64 %) and 1000 grain weight (23.26 %) whereas finger millet recorded the lowest number of productive tillers (9.41 plant-1) and ear heads (9.71 plant-1) under drought over control. Regarding the grain and straw yields of small millets subjected to water stress, it was evident from the results that the impact of water deficit at reproductive stage was prominent in little millet compared to barnyard millet and finger millet. Barnyard millet recorded the highest grain yield (18.94 g plant-1) and straw yield (42.12 g plant-1) followed by finger millet (14.48 and 33.14 g plant-1). Little millet recorded the lowest grain yield (11.31 g plant-1) and straw yield (26.01g plant-1) under drought condition.

Influence of arbuscular mycorrhiza on antioxidative system of wheat (Triticum aestivum) under drought stress

The effect of arbuscular mycorrhizal fungi (AMF) on antioxidative system in drought tolerant (WH 1025) and drought susceptible (WH 1105) wheat varieties was investigated in screen house under control and stress conditions. Mycorrhizal and non-mycorrhizal wheat (Triticum aestivum L.) plants were subjected to water stress by withholding irrigation at different stages of plant growth (i.e. jointing and heading stages). The antioxidant and antioxidative enzymes were estimated in leaves of water stressed and control plants. It was found that drought tolerant and drought susceptible varieties showed different response under drought conditions. Variety WH 1105 suffered greater damage to cellular membrane due to high level of reactive oxygen species (ROS) as indicated by superoxide radical (O.-), hydrogen peroxide (H2O2) and malondialdehyde (MDA) content under stress conditions. Antioxidative enzymes viz. superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR), catalase (CAT) and peroxidase (POX) were higher in drought tolerant variety. Antioxidative metabolites viz. ascorbic acid and glutathione content was increased in both tolerant and susceptible variety under water stress but with higher magnitude in WH 1025 than WH 1105. Results showed that under water stress conditions, mycorrhizal inoculation significantly decreased the O2 .-, H2O2, and MDA content and enhance the activities of antioxidative enzymes in both the varieties. But it was found that the activity was higher in tolerant variety than susceptible variety under water stress conditions. Hence, overall results suggest that mycorrhizal symbiosis play a vital role in enhancing the activities of antioxidative enzymes and decreasing the ROS content that enables the host plant to sustain the drought conditions.

Relationships between leaf morpho‐anatomy, water status and cell membrane stability in leaves of wheat seedlings subjected to severe soil drought

AbstractThe impact of the genotype‐specific leaf morphological and anatomical characteristics on the ability of wheat plants to preserve leaf water balance and cell membranes stability under drought stress was investigated. Seedlings of six modern semi‐dwarf (carriers of Rht, Reduced height genes) and six old tall bread wheat varieties were subjected to soil drought by withholding watering for 6 days. Morpho‐anatomical traits (leaf area, perimeter, thickness, stomata and trichome density) of daily watered (control) plants were characterized by light microscopy, scanning and image analyses. The leaf water status in both control and stressed plants was determined by measuring the relative water content (RWC). The leaf cell membranes stability in stressed plants was estimated by conductometric determination of the membranes injury index. On average, the modern semi‐dwarf varieties had less leaf area and leaf perimeter, and less dissection index, a parameter characterizing the leaf shape. Under drought stress, the modern genotypes maintained better water balance evidenced by significantly higher leaf RWC and better‐preserved the cell membranes stability supported by significantly lower Injury index. The correlations between morpho‐anatomical traits in control plants and drought tolerance‐related traits showed that the higher the leaf dissection index (i.e. more oblong leaves), the greater the water loss and the leaf membrane damages after desiccation were. The effect of shape of the evaporating surface on the water loss was modelled using wet filter paper. Similar to plant leaves, the evaporation and, respectively, water loss from paper pieces of more oblong shape (i.e. higher dissection index) was more intensive. The elucidation of the impact of the leaf shape on transpiration might contribute to better understanding of the mechanisms used by plants to maintain water reserves during drought stress and could be a basis for developing of simple and fast screening methods aiding the selection of drought tolerant genotypes.

Drought tolerance of bioenergy grass Saccharum spontaneum L. enhanced by arbuscular mycorrhizae

This study explores the effects of arbuscular mycorrhizal (AM) represented by Glomus spp. colonization on Saccharum spontaneum (L), a potential bioenergy grass subjected to drought stress under greenhouse conditions. The different treatments were: (1) watered control plants without AM association, (2) watered control plants with AM association, (3) drought stressed plants without AM association, (4) drought stressed plants. The AM association was established with Saccharum spontaneum (L) as evident from the increase in percentage of root infection and distribution frequency of vesicles. AM plants maintained a higher leaf osmotic potential and relative water content as compared to non-AM plants to counter osmotic stress in S. spontaneum. Accumulation of sugar (71% and 47% in AM and non-AM plants, respectively), proline (8.5 fold and 4 fold in AM and non-AM plants, respectively) and phenolic content (101% and 68% in AM and non-AM plants, respectively) were higher in AM plants as compared to non-AM plants on 12d of imparting drought stress. Protein (3.4 and 2.4 fold in AM and non-AM plants, respectively) and amino acid (63% and 46% in AM and non-AM plants, respectively) content were significantly increased in AM plants over non-AM plants on 8d of drought stress. Similarly, activities of SOD (superoxide dismutase), APX (ascorbate peroxidase) and GPX (guaiacol peroxidase) were higher in AM plants as compared to non-AM plants. Lipid peroxidation (83% and 129% in AM and non-AM plants, respectively) was more pronounced in non-AM plants as compared to AM plants on 12d of imparting drought stress. Reduction in total chlorophyll content (63% and 85% in AM and non-AM plants, respectively) and activities of photosystem I (33% and 42% in AM and non-AM plants, respectively) and photosystem II (27% and 57% in AM and non-AM plants, respectively) was more evident in non-AM plants as compared to AM plants. The maximum quantum efficiency of PS II primary photochemistry (Fv/Fm), potential photochemical efficiency (Fv/Fo) and the maximal fluorescence (Fm) were higher in AM plants than non-AM plants. Based on these results, it can be deduced that the AM association helps the acclimation of S. spontaneum grown in drought stress conditions. AM association aid in enhancing the inherent drought tolerance potential of S. spontaneum by maintaining better osmotic status and enhanced antioxidation which have resulted in lesser decline of total chlorophyll and photosynthetic capacity.

First Report of Verticillium Wilt on Lettuce (Lactuca sativa) in Washington Caused by Verticillium tricorpus.

Symptoms of Verticillium wilt were observed on lettuce (Lactuca sativa L.) harvested from high tunnel and open field experimental plots in annual, consecutive spring plantings in western Washington from 2010 to 2012. Leaves had v-shaped, chlorotic lesions, and yellow or brown vascular tissue was noted in the crowns. Total disease incidence increased from 0.2% in 2010 to 1.9% in 2011 and to 14.4% in 2012. Verticillium spp. obtained from infected crown tissues and cultured on half-strength potato dextrose agar medium produced yellow pigment, black microsclerotia, white mycelia, tan chlamydospores, and uniseptate conidia averaging 10.6 × 3.7 μm. Isolates were identified tentatively as Verticillium tricorpus I. (3). Three isolates, Vt.Ls.2010, Vt.Ls.2011-1, and Vt.Ls.2011-2, were evaluated for pathogenicity on 4-week-old 'Coastal Star' seedlings in two greenhouse trials. In Trial I, four replicates of two duplicate plants per each isolate, and in Trial II, five replicates of one plant per each isolate were inoculated with conidial suspensions adjusted to 2.0 × 106 and 5.0 × 106 conidia/ml, respectively. Additionally, in each trial, two sets of control treatments of five plants each were inoculated with either an isolate of V. dahliae at the same conidial concentration or with sterile water. Root tips were cut and exposed to the suspensions for 5 s, then seedlings were transplanted into Sunshine Mix #1 (SunGro Horticulture Distribution Inc., Bellevue, WA), and kept in a greenhouse at 17.7 ± 3.4°C. Plants were harvested 8 to 9 weeks post-inoculation, and symptoms were rated visually. Vt.Ls.2010, Vt.Ls.2011-1, and Vt.Ls.2011-2 caused chlorosis and vascular discoloration on 25, 13, and 13% of the plants in Trial I; and 40, 60, and 20% of plants in Trial II, respectively. V. dahliae caused similar symptoms on 25 and 40% of the plants in the two trials, respectively, but these plants had greater intensity and length of vascular discoloration compared with the three test isolates. None of the water control plants were symptomatic. All V. tricorpus isolates were recovered from inoculated plants, and colony morphologies were similar to the original isolates. The internal transcribed spacer (ITS) rDNA of isolate Vt.Ls.2010 was amplified with ITS4 and ITS6 primer sets. ITS rDNA sequences between Vt.Ls.2010 and two isolates of V. tricorpus in GenBank (Accession Nos. FJ900211 and AB353343) were 100% identical. V. tricorpus is considered a weak pathogen of lettuce crops in California (2), but authors in Japan recently reported pathogenic isolates of V. tricorpus on lettuce (4). To our knowledge, this is the first report of Verticillium wilt caused by V. tricorpus in Washington. Lettuce is the number two crop grown in high tunnels in the United States (1), and cropping lettuce continuously in them can increase the risk of this and other soilborne pathogens.

Three cycles of water deficit from seed to young plants of Moringa oleifera woody species improves stress tolerance

The main objective of this study was to assess whether recurring water stress occurring from seed germination to young plants of Moringa oleifera Lam. are able to mitigate the drought stress effects. Germination, gas exchange and biochemical parameters were analysed after three cycles of water deficit. Young plants were used 50 days after germination under three osmotic potentials (0.0, -0.3 and -0.4 MPa). For each germination treatment, control (irrigated) and stressed (10% of water control) plants were compared for a total of six treatments. There were two cycles of drought interspersed with 10 days of rehydration. The young plants of M. oleifera showed increased tolerance to repeated cycles of drought, maintaining high relative water content (RWC), high water use efficiency (WUE), increased photosynthetic pigments and increased activity of antioxidant enzymes. There was rapid recovery of the photosynthetic rate during the rehydration period. The stressed plants from the -0.3 and -0.4 MPa treatments showed higher tolerance compared to the control plants. The results suggest that seeds of M. oleifera subjected to mild water deficit have had increased the ability for drought tolerance when young plant.

Galactinol Is a Signaling Component of the Induced Systemic Resistance Caused by Pseudomonas chlororaphis O6 Root Colonization

Root colonization by Pseudomonas chlororaphis O6 in cucumber elicited an induced systemic resistance (ISR) against Corynespora cassiicola. In order to gain insight into O6-mediated ISR, a suppressive subtractive hybridization technique was applied and resulted in the isolation of a cucumber galactinol synthase (CsGolS1) gene. The transcriptional level of CsGolS1 and the resultant galactinol content showed an increase several hours earlier under O6 treatment than in the water control plants following C. cassiicola challenge, whereas no difference was detected in the plants without a pathogen challenge. The CsGolS1-overexpressing transgenic tobacco plants demonstrated constitutive resistance against the pathogens Botrytis cinerea and Erwinia carotovora, and they also showed an increased accumulation in galactinol content. Pharmaceutical application of galactinol enhanced the resistance against pathogen infection and stimulated the accumulation of defense-related gene transcripts such as PR1a, PR1b, and NtACS1 in wild-type tobacco plants. Both the CsGolS1-overexpressing transgenic plants and the galactinol-treated wild-type tobacco plants also demonstrated an increased tolerance to drought and high salinity stresses.

“Identification and expression of six drought-responsive transcripts through differential display in desi cotton (Gossypium arboreum)”

There is not enough information available on drought-modulated gene(s) in Gossypium arboreum, which can be a valuable gene pool for improving modern cotton cultivars. In the present work differential display reverse transcriptase PCR (DDRT) was used to compare overall differences in gene expression between water stressed and control plants. By screening 93 primer-pair combinations DDRT technique resulted in up-regulation of 30 cDNA transcripts. Through reamplification and quality control assay 10 cDNA transcripts appeared false positive. The remaining 20-cDNA transcripts were extracted from the gel, reamplified, cloned and sequenced. Homology search revealed that 6 transcripts showed significant homology with known genes. Real-time RT-PCR showed that among 6 transcripts 5 showed significant over expression in water stressed leaves as compared to control. This is an important finding since there are only few reports of universal stress protein and transposable elements are available in plants but none in cotton under drought condition.

Stalk Rot of Sugar Beet Caused by Fusarium solani on the Pacific Coast.

In 2006, symptoms of stalk blight (2) were observed on sugar beet (Beta vulgaris L.) plants from roots produced in Oregon that were being grown for seed production in a greenhouse in Salinas, CA using Salinas Valley soil. Symptoms included vascular and cortical browning, necrosis, and death of seed stalks. Isolations were made from the edge of stalk lesions and the crown. In addition to Fusarium oxysporum, the known cause of stalk blight (2), two isolates of F. solani were identified by morphology. For pathogenicity tests, sugar beet plants (FC606 [4]), grown in pasteurized potting mix and induced to flower by exposure at 4 to 7°C for 90 days (1) were used. Bolting plants were maintained in a greenhouse at 24 to 27°C. A 100-μl drop of a spore suspension (104 spores per ml) of each Fusarium isolate was placed on the surface of the seed stalk. The plant was stabbed through the drop with a sterile 18-gauge needle so that the drop was taken into the plant by hygroscopic pressure. Positive and negative control treatments were a stalk blight isolate of F. oxysporum from an Oregon seed production field and sterile water, respectively. Three plants were inoculated per isolate. Each inoculation site was wrapped loosely in Parafilm for 1 week to maintain a high humidity level around the site of inoculation, and seed stalks were covered in cloth bags (1). After 1 week, the Parafilm was removed and plants were examined weekly for symptoms. At 4 weeks, lesion size was measured. After 5 weeks, sections were taken from the seed stalk around the site of inoculation, surface disinfested with 0.5% NaOCl, and plated on potato dextrose agar to confirm the presence of the pathogen. The experiment was done twice. One of the two isolates of F. solani caused dark brown lesions on all inoculated seed stalks. On one plant, at 4 weeks after inoculation when the bag was being removed for observation, the seed stalk broke at the site of inoculation because of a spreading, brown lesion at the site. No lesions were observed on the water control plants. Brown lesions were observed on seed stalks inoculated with the known stalk blight isolate. Lesions were significantly (P = 0.001) larger with F. oxysporum than with F. solani when measured at 4 weeks (mean of 6.3 cm versus 2.2 cm, respectively). Lesions caused by F. solani showed a dark discoloration through the cortical tissue, as opposed to those caused by F. oxysporum, for which most of the initial discoloration was in the vascular bundles and epidermis. Fusarium isolates recovered from inoculated plants were morphologically similar to the isolates used for inoculation. Fusarium spp. were not isolated from the water control plants. While some F. solani isolates cause seedling or mature root disease in sugar beet (3), to our knowledge, this is the first report of a Fusarium species other than F. oxysporum causing a rot of sugar beet stalks.

Morphological and physiological responses of two coffee progenies to soil water availability

Drought is a major environmental constraint affecting growth and production of coffee. The effects of water supply on growth, biomass allocation, water relations, and gas exchange in two coffee progenies representing drought-tolerant (Siriema) and drought-sensitive (Catucaí) genotypes were compared. They were grown in 12-L pots until 4-months old, when they were submitted to two watering treatments for 60 d: plants receiving either 100% transpired water (control plants) or a fraction (about 40%) of the amount of water transpired by control plants (drought-stressed plants). Under control conditions, Siriema grew faster than Catucaí. Regardless of the watering regimes and progenies, relative growth rate (RGR) was positively correlated both with net assimilation rate (NAR) and long-term water-use efficiency (WUE), but not with differences in biomass allocation. Both progenies responded to drought stress through (i) similar decreases in both RGR and NAR with marginal, if any, changes in allocation; (ii) decreases in leaf water potential, which occurred to a greater extent in Catucaí than in Siriema, even though they have showed similar abilities to adjust osmotically and elastically; (iii) similar reductions in net photosynthesis due mainly to nonstomatal factors; and (iv) decreases in transpiration rate coupled with increased long-term WUE. However, the lower transpiration rate and the higher long-term WUE as found in Siriema relative to Catucaí under control conditions persisted under drought conditions. Overall, the major differences between these progenies were largely associated with differences in plant water use, which was likely related to the improved water status of Siriema. The possible implications of selecting coffee genotypes for high WUE are discussed.

Occurrence of Fusarium Wilt on Canola Caused by Fusarium oxysporum f. sp. conglutinans in Argentina.

Canola (Brassica napus) is a developing oleaginous crop grown commercially in Argentina, primarily in the southeastern region of Buenos Aires Province. Since 2002, plants exhibiting symptoms of wilt and xylem discoloration were observed in canola plants in experimental field plots located at the University of Buenos Aires, Buenos Aires, Argentina. Average disease incidence in 5- to 6-month-old canola cultivars developed in different countries was 18% (range = 9 to 27%). Disease symptoms that included yellowing, wilting, stunting, and necrosis of leaf tissue and suppressed root development appeared in irregular-shaped patches following the rows of plants. The first symptom observed was leaf yellowing followed by an irregular, brown necrosis of the leaf margins. Lesions coalesced to form large necrotic areas that led to severe defoliation beginning with the lower leaves. As the disease developed, a pale brown discoloration girdled the stems that progressed from the basal tissues to the apex. Affected plants were stunted and had small pods with no seeds. Diseased plants eventually collapsed and died. From June to July 2003, six samples consisting of five affected plants per sample were randomly collected from experimental field plots. Pieces (1 cm long) of disease basal stem tissue were thoroughly washed, surface sterilized in 1% sodium hypochlorite for 1 min, rinsed in sterile distilled water, blotted dry on sterile Whatman's filter paper, and incubated on potato dextrose agar in the dark at 26°C for 10 days. Ten resulting colonies were examined microscopically and identified as Fusarium oxysporum Schlechtend.:Fr. f. sp. conglutinans (Wollenweb.) W.C. Snyder & H.N.Hans. (3). Pathogenicity tests for three single-spore isolates of the fungus were performed on 6-week-old canola plants of cvs. Impulse, Master, Mistral, Monty, Rivette, and Trooper. Koch's postulates were completed for each isolate by dipping the roots of seedlings in a conidial suspension (2 times; 105 conidia per ml) for 15 min. Plants were repotted in a sterilized soil mix (soil/sand, 2:1). The experiment, which included five inoculated plants and three noninoculated (roots dipped in sterile distilled water) control plants for each cultivar, was conducted in a greenhouse at 23 to 25°C and 75% relative humidity with no supplemental light. Characteristic symptoms, identical to the original observations, developed within 14 days after inoculation on 100% of the inoculated plants for all three isolates. The pathogen was successfully reisolated from internal diseased stem tissue in all instances. Symptoms included stunted seedlings, leaf necrosis, and external stem discoloration. None of the control plants developed disease. The experiment was repeated once with similar results. F. oxysporum f. sp. conglutinans, which has been reported to cause disease in canola in Canada (1) and the United States (2), represents a serious threat to the main canola cultivars grown in Argentina. To our knowledge, this is the first report of canola wilt incited by F. oxysporum f. sp. conglutinans in Argentina.

Physiological Responses of Pepper to Salinity and Drought

Production of vegetable crops can be limited by saline irrigation water. The variability of crop salt tolerance under different environmental conditions requires species-specific and environment-specific field evaluations of salt tolerance. Data on field performances of vegetable crops grown on soils that have been irrigated with saline water for many years are lacking. In this study we analyzed the long-term effect of irrigation with saline water on soil properties and on responses of field-grown pepper (Capsicum annuum L.) plants in these soils. Yield, gas exchanges, water relations, and solute accumulation were measured in plants grown under three different irrigation treatments: a nonsalinized control (ECw = 0.5 dS·m-1) and two concentrations of commercial sea salt, corresponding to ECw of 4.4 and 8.5 dS·m-1, respectively. In addition, a nonwatered drought stress treatment was included. Irrigation water with an EC of 4.4 dS·m-1 resulted in 46% reduction in plant dry weight (leaves plus stem) and 25% reduction in marketable yield. Increasing the electrical conductivity of the irrigation water to 8.5 dS·m-1 caused a 34% reduction in plant dry weight and a 58% reduction in marketable yield. Leaf and root cellular turgor and net CO2 assimilation rates of leaves in salt-stressed plants decreased along with a reduction in leaf area and dry matter accumulation. High concentrations of Na+ and Cl- in the irrigation water did not significantly alter the level of K+ in leaves and fruit. In contrast, drought stressed plants had higher concentrations of leaf K+ compared to well watered control plants. These results indicate that Na+ and K+ may play similar roles in maintaining cellular turgor under salinity and drought stress, respectively. The regulation of ion loading to the shoots was most likely functionally associated with physiological modifications of the root/shoot ratio that was substantially smaller in salinized vs. drought stressed plants. From an agronomic perspective, irrigation with moderately saline water (4.4 dS·m-1) it is recommendable, compared to no irrigation, to obtain an acceptable marketable yield in the specific environment considered.

Phenological and physiological responses of heavily dusted creosote bush (Larrea tridentata) to summer irrigation in the Mojave Desert

In the Mojave Desert, numerous human activities damage soil structure, often resulting in increased wind erosion and deposition of dust on plant leaves. Especially susceptible to dust coating are the resinous, evergreen leaves of the dominant creosote bush ( Larrea tridentata ) wherever these shrubs grow along unpaved roads and trails and at the margins of dry playas. A yearlong field study was conducted, begun during summer drought, to investigate effects of summer irrigation on heavily dusted plants of L. tridentata . Plant water relations, quantitative phenology, and gas exchange were monitored for sets of five plants under three conditions: dusted nonirrigated, dusted irrigated, and undusted nonirrigated (control). Following watering, dusted irrigated plants experienced rapid shoot growth and had significantly higher predawn shoot water potentials (Ψ = −2.5 MPa) than dusted nonirrigated plants (−5.5 MPa), and midday values were −3.1 to −4.1 MPa versus −5.5 to −6.4 MPa, respectively. For dusted plants, irrigation resulted in marked increases of assimilation (14.0 versus 1.24μmol m −2 s −1 ), stomatal conductance (0.19 versus 0.03 mol m −2 s −1 ), and linear growth, and accompanying new growth the old, dusted leaves were mostly abscised. All measures of water-use efficiency (WUE) were higher in controls than in dusted plants, and during the summer, dust was associated with a 40–90% reduction in shoot growth. Nonirrigated controls showed much greater water stress than dusted irrigated plants, but when irrigation was discontinued, the formerly irrigated plants rapidly converged on the physiological and phenological characteristics of not watered control plants. Dust deposition may reduce plant carbon gain by decreasing WUE and impairing tolerance of water stress. This experiment demonstrated that creosote bushes can recover rapidly from acute heavy dust deposition if irrigation, simulating heavy summer rainfall, is provided, and these results may be useful in measures to minimize the ecological impact of human activities in desert ecosystems.

Plant Morphological and Biochemical Responses to Field Water Deficits

Activity of glutathione reductase has been related to stress tolerance; however, these enzyme assays are generally conducted at 25 degrees C. Foliage temperature varies greatly in the field in response to soil water availability and ambient conditions and this may affect enzyme response. This study was conducted to determine the effect of changing foliage temperature on glutathione reductase activity of wheat under field conditions. Wheat leaf glutathione reductase was purified and the temperature response of the enzyme was determined at 2.5 degrees C intervals between 12.5 and 45 degrees C. These data, in conjunction with continuous measurements of field-grown wheat foliage temperatures, were used to compare the temperature-related changes in potential glutathione reductase activities in water stressed and control plants. Assuming saturating substrate levels, the results indicate that early in the season the daily potential enzyme activity of the irrigated and stressed plants could never have reached the daily activity predicted from the 25 degrees C (room temperature) measurements. Later in the season, the daily potential activity of the irrigated plants was lower, and the daily potential activity of the stressed plants was higher, than the activities predicted from the 25 degrees C determinations. These results suggest that a better understanding of the regulation of plant metabolism will be obtained by linking continuous temperature measurements of plant foliage with enzyme responses to temperature.

Water Use Efficiency of Cassava. I. Effects of Air Humidity and Water Stress on Stomatal Conductance and Gas Exchange1

Measurements of CO2 and H2O exchange of attached cassava (Manihot esculenta Crantz) leaves in water stressed and control plants of cultivars M Col 90 and M Col 88 were made at various leaf‐air vapor pressure differences (VPD) (0.8 to 4.5 kPa). Apparent photosynthesis and transpiration were sharply reduced by increase in VPD above 1.8 to 2.0 kPa in both stressed and nonstressed plants. This trend coincided with changes in leaf conductance over similar ranges of VPD. The rapid closure of stomata in dry air was independent of bulk leaf water potential (ψl). Furthermore, the response was completely reversible in humid air after short exposure to dry air, suggesting a direct response to changes in air humidity. Water use efficiency decreased as VPD increased over the range of 1 to 4 kPa. The significance of the Stomatal response to humidity is discussed in relation to WUE in long dry periods.