Development Of Foliar Symptoms Research Articles

The potential role of <i>Aureobasidium pullulans</i> in the development of foliar symptoms of Esca disease in grapevine

Esca belongs to the group of grapevine trunk diseases - fungal diseases present worldwide in all wine-growing regions. Some aspects of the disease, like the development of external symptoms, have still not been completely discovered and are believed to be affected by several factors, including interactions within the vine microbiome. The examination of the occurrence of the yeast-like fungus Aureobasidium pullulans in the healthy wood of Esca-diseased grapevines via both isolation and qPCR measurements showed a positive correlation between its abundance and the severity of foliar symptoms, suggesting the contribution of this fungus to Esca pathogenesis via an indirect action. In vitro confrontation tests revealed antagonistic interaction between A. pullulans and the Esca pathogen Phaeomoniella chlamydospora. Mutual growth inhibition and the induction of asexual sporogenesis were observed for both fungi without cytotoxic effects. In planta confrontation tests revealed that A. pullulans in combination with P. chlamydospora can lead to severe foliar damage in a strain-dependent manner. This phenomenon could be explained by the altered metabolism of the Esca pathogen in the presence of A. pullulans, or by the cumulative/synergistic effects of the secreted polysaccharides and/or proteins of the two fungi. The present study shows the importance of microbial interactions in the development of plant diseases, highlighting that even a non-pathogenic microorganism can act as a disease-enhancer.

Effect of Different Foliar Fertilizer Applications on Esca Disease of Grapevine: Symptom Expression and Nutrient Content in the Leaf and Composition of the Berry

Esca disease, the most common grapevine wood disease in Europe, causes yield losses correlated with the foliar symptoms’ expression. In two vineyards located in the Abruzzo Region of Italy, each of which were investigated for esca symptoms after 1994, different applications of macro- and microelements were performed in two consecutive growing seasons. The main aim of the work consisted of verifying the effects of the fertilizer applications on the foliar symptoms’ expression, in order to deepen knowledge of the nature of the symptom, which is still unclear. For each treatment, in each year, the leaf content of macro- and microelements and the composition of berries and musts were assessed. The effects of these applications on vegetative growth and yield quantity were also verified. The trials were carried out on symptomatic, asymptomatic and apparently healthy vines. All applications, in particular those with microelements, increased the foliar symptoms’ expression, and a greater vegetative growth was detected only in vines treated with NPK fertilizers. The symptoms’ increase was always associated in the leaf with a decrease of calcium, and to a lesser extent, magnesium, reinforcing the hypothesis of the plant’s hypersensitive reaction in the development of foliar symptoms, given the role of calcium in the defense response. The vineyards were in nutritional balance regardless of the fertilizer applications. The general increase in foliar symptoms and the decrease in sugars in the musts of asymptomatic treated vines underlined the importance of the vegetative-productive balance, in Esca infected vineyards especially, in order to limit the symptoms’ expression and the decrease in yield.

Diversity in virulence and metalaxyl-m sensitivity of Phytophthora rubi isolates has implications for raspberry root rot and wilting complex management

Raspberry root rot and wilting complex (RRWC), primarily caused by Phytophthora rubi, is the most important disease for raspberry producers in Canada. There is currently little to no information on phenotypic diversity or sensitivity to metalaxyl-m of P. rubi isolates affecting raspberries in Canada. This study was conducted to understand the diversity in virulence and sensitivity to metalaxyl-m of P. rubi populations collected from diverse raspberry cultivars, fields, and locations during 2018–2020. All isolates of P. rubi (n = 24) were pathogenic on ‘Chemainus’ raspberry and showed significant variation (P < 0.05) in days to first foliar symptom development (DFSD) and progress in wilting index, as indicated by relative areas under disease progress curves (rAUDPC). Principal component analyses identified that root rot index, rAUDPC, DFSD, and percentage reduction in root length and weight were important variables in assessing pathogen virulence. Metalaxyl-m sensitivity was assessed using a radial growth assay and effective concentrations that inhibited 50% of mycelial growth (EC50) were determined for 44 isolates. The EC50 values ranged from 0.035–4.71 µg mL−1 with a mean value of 0.793 µg mL−1. About 61% of the isolates were very sensitive (EC50 values <0.20 µg mL−1), 25% of isolates were considered sensitive (EC50 values of 0.21–1.18 µg mL−1), and 14% of isolates showed reduced sensitivity to metalaxyl-m (EC50 value range of 1.19–4.71 µg mL−1). Knowledge of pathogen phenotypic diversity will be useful for raspberry breeders in screening genotypes against RRWC and the results of the fungicide sensitivity study will be useful in developing management strategies for RRWC.

Grapevine Leafroll-Associated Virus 3 Genotype Influences Foliar Symptom Development in New Zealand Vineyards.

Grapevine leafroll disease (GLD) constrains wine production worldwide. In New Zealand, the main causal agent of GLD is grapevine leafroll-associated virus 3 (GLRaV-3). To control GLD, an integrated management program is used and includes removing (roguing) GLRaV-3-infected vines from the vineyard. The classical foliar symptoms from virus-infected red-berry cultivars are leaves with dark red intervein, green veins, and downward rolling of margins. Growers use these phenotypic cues to undertake visual symptom identification (VSI) for GLD. However, the influence of the known large genetic variation among GLRaV-3 isolates on the foliar symptoms from different grapevine cultivars remains undescribed, especially in cool-climate growing environments, such as New Zealand. Over three vintages (2015, 2016, and 2017), VSI for GLD was undertaken at three field sites in New Zealand (Auckland, Hawke’s Bay, and Marlborough), each including four cultivars (Merlot, Pinot noir, Sauvignon blanc, and Pinot gris) infected with three GLRaV-3 genotypes (Groups I, VI, and X) or GLRaV-3-uninfected control plants. Throughout this study, no visual symptoms were observed on white-berry cultivars infected with GLRaV-3. For red-berry cultivars, the greatest variability in observed foliar symptoms among regional study sites, cultivars, and GLRaV-3 genotypes was observed early in the growing season. In particular, Group X had significantly delayed symptom expression across all three sites compared with Groups I and VI. As the newly infected, young vines matured in years 2 and 3, the GLRaV-3 genotype, cultivar, region, and environmental conditions had minimal influence on the accuracy of VSI, with consistently high (>95%) within-vintage identification by the end of each vintage. The results from this study strongly support the use of VSI for the GLD management of red-berry cultivar grapevines, Merlot and Pinot noir, as a reliable and cost-effective tool against GLD.

Physiological responses of ‘Italia’ grapevines infected with Esca pathogens

Physiological features were examined of a 20-year-old Vitis vinifera ‘Italia’ table grape vineyard cropped in Apulia, Italy. Healthy vines with no foliar symptoms and any indications of wood or berry alterations, vines with natural wood infections by Phaeoacremonium minimum (syn. P. aleophilum) and Phaeomoniella chlamydospora showing brown wood streaking symptoms, and vines naturally infected with P. minimum, P. chlamydospora and Fomitiporia mediterranea with brown wood streaking and white rot symptoms, were surveyed. Bleeding xylem sap, collected at bud-break from healthy vines showed the greatest total ascorbic acid level, while vines with brown wood streaking and white rot had the greatest viscosity coefficient, glutathione concentration, and plant growth regulator activities. Compared to healthy vines, leaves of wood affected vines, sampled during the unfolded leaf, fruit setting, cluster closing and bunch ripening vine growth stages, had reduced fresh and dry weights, total chlorophyll concentrations, and increased leaf surface area. Low ascorbic acid and reduced glutathione concentrations, weak redox state, and moderate levels of dehydroascorbic acid and oxidized glutathione were also detected in these vines. Analyses also detected reduced activities of dehydroascorbate reductase, ascorbate free radical reductase and glutathione reductase in diseased vines. The cell membrane damage, associated with lipid peroxidation, was coupled with high hydrogen peroxide concentrations. These changes could contribute to the cell death of leaves and foliar symptom development. The ascorbate-glutathione cycle supports grapevine susceptibility to Esca complex-associated fungi.

Potential host range and the effect of temperature on the pathogenicity of Phytophthora pseudocryptogea and its close relatives

Phytophthora pseudocryptogea, a member of clade 8a of the Phytophthora phylogenetic tree, was formally described in 2015 as a close relative of P. cryptogea and P. erythroseptica. The potential host range of this newly described species and its close relatives was investigated in this study. Thirty species of herbaceous plants and nine species of woody plants were inoculated and monitored for development of foliar symptoms and root rots. Also, we inoculated the detached twigs of 21 tree species, fruit of six plant species, tubers of potato, and roots of carrot and sugar beet. Our results showed P. pseudocryptogea, P. cryptogea and P. erythroseptica had different potential host ranges, though all were pathogenic on some plant species such as pistachio, tomato, potato, eggplant, spinach and clover. Additionally, some plant species were able to discriminate P. pseudocryptogea from P. cryptogea including cucumber, melon, watermelon, alfalfa, soybean, cabbage, green pepper, and rice. These plants showed susceptibility to P. pseudocryptogea and resistance to P. cryptogea. Different species showed different virulence levels on detached tree twigs; however, almost all species caused severe rot on all tested fruits, tubers and roots. Investigation of the effects of root-zone temperature on the virulence of species showed that a temperature of 20 °C in increased root rot and root colonization in P. pseudocryptogea and P. erythroseptica, while P. cryptogea caused the most root rot and colonization at 25 °C.

Multi-location evaluation of fluopyram seed treatment and cultivar on root infection by Fusarium virguliforme, foliar symptom development, and yield of soybean

A study was conducted in five American states and Ontario, Canada, in 2015 and 2016 to determine the effects of fluopyram seed treatment and cultivar on the root rot and foliar phases of sudden death syndrome (SDS) of soybean. Three seed treatments were evaluated: (1) base treatment (control) containing prothioconazole + penflufen + metalaxyl (0.019 mg a.i./seed) + metalaxyl (0.02 mg a.i./seed) + clothianidin + Bacillus firmus I-1582 (0.13 mg a.i./seed), (2) base treatment + fluopyram (0.15 mg a.i./seed), and (3) base treatment + fluopyram (0.075 mg a.i./seed). Three soybean cultivars, categorized as susceptible, moderately resistant and resistant were planted at each location. Both rates of fluopyram reduced root rot and foliar disease index (FDX) and increased yield compared with the base treatment. The two rates of fluopyram did not differ for reducing root rot or FDX, but yield was greater with the higher versus lower rate. Fluopyram reduced root colonization by Fusarium virguliforme as measured with quantitative PCR in one of two study years. Yield was not correlated with root rot at the V2, but was negatively correlated with root rot at the R4/R5 growth stage and with FDX. Root rot at R4/R5 was positively correlated with FDX. A yield benefit to fluopyram was found in a location where root rot but no foliar symptoms were observed. These findings suggest that fluopyram seed treatment can reduce the root rot and foliar phases of SDS, and both phases play an important role in yield and should be managed accordingly.

Identification and confirmation of hotspot areas and management ofroot (wilt) disease in coconut

Phytoplasma malady associated with root (wilt) disease in coconut is a serious threat and causes heavy economic loss to the coconut farmers in Kerala and Tamil Nadu in India. A roving survey was carried out during 2016–2019 to assess the disease incidence in Coimbatore, Tirupur, Dindigul, Theni, Tenkasi, Tirunelveli and Kanyakumai districts. From the results, Cumbum block in Theni district, Tenkasi block in Tirunelveli (Tenkasi) district and Kuruthancode block in Kanyakumari district were identified as hot spot area for root (wilt) incidence. The disease is generally recognized based on leaf symptoms, but the development of foliar symptoms in coconut palms is very slow and there is a time lag between infection and symptom expression. To the confirmation of the latent infection of root (wilt) affected palms, Direct Antigen Coated (DAC) indirect ELISA has been standardized earlier. Since this ELISA system requires 24 hours, a modified protocol was developed using leaf disc as test antigen that was simple, rapid and provide results within 7 hours. The test yielded 98% more sensitivity with respect to the visual observations. The modified protocol is used for the routine screening of coconut palms for selection of disease-free mother palms and for developing root (wilt) resistant varieties. Besides this, the test is used for confirmation of root (wilt) disease in the early stages of the infection and in newly affected areas. The result of the field experiment on root (wilt) management revealed that the soil application of microbial consortia (Trichoderma viride (Tv)+ Pseudomonas fluorescens (Pfi)+ Bacillus. subtilis) at 100 + FYM 5 kg + phosphobacteria at 200 g at quarterly intervals along with copper sulphate at the concentration of 75 g at three months intervals was effective in reducing the disease incidence from 13.33 to 8.13% after two years of treatment.

Identification of Multiple Phytotoxins Produced by Fusarium virguliforme Including a Phytotoxic Effector (FvNIS1) Associated With Sudden Death Syndrome Foliar Symptoms

Sudden death syndrome (SDS) of soybean is caused by a soilborne pathogen, Fusarium virguliforme. Phytotoxins produced by F. virguliforme are translocated from infected roots to leaves, in which they cause SDS foliar symptoms. In this study, additional putative phytotoxins of F. virguliforme were identified, including three secondary metabolites and 11 effectors. While citrinin, fusaric acid, and radicicol induced foliar chlorosis and wilting, Soybean mosaic virus (SMV)-mediated overexpression of F. virguliforme necrosis-inducing secreted protein 1 (FvNIS1) induced SDS foliar symptoms that mimicked the development of foliar symptoms in the field. The expression level of fvnis1 remained steady over time, although foliar symptoms were delayed compared with the expression levels. SMV::FvNIS1 also displayed genotype-specific toxicity to which 75 of 80 soybean cultivars were susceptible. Genome-wide association mapping further identified three single nucleotide polymorphisms at two loci, where three leucine-rich repeat receptor-like protein kinase (LRR-RLK) genes were found. Culture filtrates of fvnis1 knockout mutants displayed a mild reduction in phytotoxicity, indicating that FvNIS1 is one of the phytotoxins responsible for SDS foliar symptoms and may contribute to the quantitative susceptibility of soybean by interacting with the LRR-RLK genes.

Quantitative trait loci underlying host responses of soybean to Fusarium virguliforme toxins that cause foliar sudden death syndrome.

Soybean deploys multiple genetic mechanisms to confer tolerance to Fusarium virguliforme toxins. This study revealed that F. virguliforme culture filtrates could be used in mapping QTL underlying foliar SDS resistance. Sudden death syndrome (SDS) is a major soybean disease throughout most of the soybean growing regions in the world including the United States. The disease is caused by the fungal pathogen, Fusarium virguliforme (Fv). The fungus produces several toxins that are responsible for development of interveinal leaf chlorosis and necrosis, which are typical foliar SDS symptoms. Growing of resistant cultivars has been the most effective method in controlling the disease. The objective of the present study was to identify quantitative trait loci (QTL) underlying host responses of soybean to Fv toxins present in culture filtrates. To accomplish this objective, two recombinant inbred line (RIL) populations, AX19286 (A95-684043 × LS94-3207) and AX19287 (A95-684043 × LS98-0582), segregating for SDS resistance were evaluated for foliar symptom development by applying two screening protocols, the stem cutting and the root feeding assays. The AX19286 population revealed two major and seven minor QTL for SDS resistance. In the AX19287 population, we identified five major QTL and three minor QTL. The two QTL mapped to Chromosome 7 [molecular linkage group (MLG) M] and Chromosome 20 (MLG I) are most likely novel, and were detected through screening of the AX19287 population with stem cutting and root feeding assays, respectively. This study established that Fv culture filtrates could be employed in mapping QTL underlying foliar SDS resistance. The outcomes of the research also suggest that multiple genetic mechanisms might be used by soybean to overcome the toxic effects of the toxins secreted by the pathogen into culture filtrates.

Early root infection and damage in Huanglongbing disease development

Huanglongbing in grove trees is initially identified by foliar symptoms, most commonly blotchy mottle.&nbsp; Detection of Candidatus Liberibacter asiaticus (Las) in leaf tissue by qPCR early in disease development is usually limited to symptomatic leaves and proximal young leaves.&nbsp; Over multiple years, disease symptoms spread to the rest of the canopy.&nbsp; Although Las has been detected in root tissue, the decline of roots has been assumed to happen later in disease development when photosynthate production and transport have been significantly diminished in the tree canopy.&nbsp; Observations of initial spread of Las from the bud-inoculation site in the trunk of 1-yr-old potted trees have revealed that Las is frequently detectable in roots months before detection of Las in leaves and foliar symptom development.&nbsp; Even after symptom development Las is more evenly distributed in root tissue than in the canopy.&nbsp; Preliminary evidence suggests that Las is also more evenly distributed in roots of grove trees.&nbsp; Asymptomatic 9 year old grove trees with root Las infection had 26-41% lower root density than asymptomatic trees without detectable root Las.&nbsp; The loss of root density was independent of Las detection in leaves.&nbsp; Root loss precedes carbohydrate starvation as evidenced by root starch concentrations, suggesting the bacteria may play a more active role in root loss than phloem plugging.&nbsp; These results suggest that early invasion of roots by Las leads to root decline before the appearance of foliar symptoms and is likely the cause of larger than expected yield reduction on trees with limited foliar symptoms.

The importance of the infected seed tuber and soil inoculum in transmitting Potato mop‐top virus to potato plants

Potato mop‐top virus (PMTV), the cause of spraing in potato tubers, is transmitted by Spongospora subterranea, the cause of powdery scab, and by planting infected seed tubers. This study was undertaken to determine the relative importance of these sources of infection in seed potato production in Scotland. The transmission of PMTV from tested seed tubers to daughter plants was examined over 2 years and six cultivars. The development of foliar symptoms varied with year and cultivar. Infection of daughter tubers derived from PMTV‐infected seed tubers was more prevalent on plants affected by foliar symptoms than those without symptoms. The rate of transmission of PMTV from infected seed tubers to daughter tubers ranged from 18 to 54%. Transmission was affected by cultivar and by origin of seed tubers used for a cultivar, but not by a cultivar's sensitivity to PMTV infection. The incidence of PMTV in daughter tubers of cv. Cara grown from seed potatoes from one source (common origin) by more than 25 seed producers was examined over two successive generations. The incidence of PMTV in daughter tubers was not correlated with that in the seed tubers but appeared to be strongly associated with soil inoculum. The incidence of PMTV was correlated with powdery scab in those crops in which both were present. There was some evidence from soil tests conducted in 2006 using a tomato bait plant and real‐time RT‐PCR that planting PMTV‐infected seed potatoes could increase the risk of introducing the virus into land not infested by PMTV.

Host Specificity and Variations in Aggressiveness of North Carolina Isolates of Phytophthora cryptogea and P. drechsleri in Greenhouse Ornamental Plants.

Three isolates of Phytophthora cryptogea and three isolates of P. drechsleri were evaluated for host specificity and variations in aggressiveness on eight floriculture crops commonly grown in North Carolina. Plants were monitored for the development of foliar symptoms stemming from root rot caused by Phytophthora. No isolates of P. cryptogea or P. drechsleri had the same host range, though all P. cryptogea isolates caused foliar symptoms on gerbera daisy and annual stock, and all P. drechsleri isolates incited disease on osteospermum. No African marigold plants developed foliar symptoms of Phytophthora root rot. All P. cryptogea isolates caused foliar symptoms on at least three tested plant species. Isolates of P. drechsleri varied from highly specific and only causing disease on a single host species to relatively nonspecific and causing foliar symptoms on the majority of host plants. This is the first report of host specificity of ornamental isolates of P. drechsleri, and several of the tested plants are not reported hosts of P. drechsleri. Differences in isolate aggressiveness were found and depended on plant species; overall, isolates of P. drechsleri were more aggressive than P. cryptogea isolates. Isolations from nonsymptomatic inoculated plants suggest that these Phytophthora spp. could be moved between growers or from growers to consumers on nonsymptomatic infected ornamentals.

New Insights into Esca of Grapevine: The Development of Foliar Symptoms and Their Association with Xylem Discoloration.

A new study on the development of foliar symptoms of esca was carried out from 2004 to 2006 in five mature vineyards in Aquitaine, France. Symptoms were monitored for severity and changes over time. Initial foliar symptoms were characterized by the presence of drying zones or discolorations (reddening or yellowing), which are symptoms that have also been attributed to Black Dead Arm (BDA). Then, the less-severely affected leaves persisted throughout the summer and developed into typical "tiger-stripe" symptoms of esca. The most severely symptomatic leaves fell soon after symptoms appeared. Severely diseased vines showed typical apoplectic or acute forms of esca that did not differ from the severe BDA forms. The appearance of leafsymptomatic vines increased uniformly over time, reaching a maximum incidence by the end of July. A second survey in 41 European and Lebanese vineyards showed that longitudinal discolorations were visible under the bark of 95% of the vines showing foliar esca symptoms. These wood symptoms, also previously attributed to BDA, appeared as xylem orange-brown stripes. Thus, foliar symptoms of esca showed transitory phases which overlapped with some BDA descriptions. Most of these symptoms, in the west-palearctic regions that were investigated, were commonly associated with the presence of one or several xylem discolorations.

Plant Age Affects Root Infection and Development of Foliar Symptoms of Soybean Sudden Death Syndrome.

Soybean sudden death syndrome is characterized by root rot followed by the development of foliar symptoms. However, it is not known how time of infection affects disease severity. Soybean plants were inoculated at 0, 4, 7, 14, 21, 28, and 35 days after planting (DAP) by drenching potting media with conidia of Fusarium virguliforme, then incubating in growth chambers at 17°C for 7 days followed by 24°C for 31 days. Root rot and severity of foliar symptoms were assessed 18 and 38 days after inoculation (DAI). Root rot developed on plants inoculated at all ages but plants inoculated at seed stage (0 DAP) had the highest (P < 0.01) root rot severity (>90%). At 38 DAI, foliar symptoms were severe (>80%) on plants inoculated at 0 DAP but did not develop on plants inoculated at all other ages. Xylem colonization by F. virguliforme was more frequent in plants inoculated at 0 DAP than on plants inoculated at later stages. The results of this study suggest that soybean roots become less susceptible to xylem colonization and the subsequent development of foliar symptoms as plants mature. Therefore, practices aimed at protecting seed and seedling roots from infection may improve soybean sudden death management.

Systemic damage in leaf metabolism caused by esca infection in grapevines

Background and Aims: Esca is a devastating wood-decay disease that affects major grape-producing areas around the world. Xylem pathogenic fungi produce a systemic effect on the development of foliar symptoms and glutathione metabolism in presence of oxidative species. This study focuses on sugar variations in Vitis vinifera leaves, in particular in early events occurring in esca. Methods and Results: Biochemical, cytochemical and molecular methods showed that in esca-infected grapevines, photosynthetic pigment content did not vary significantly in leaves from apparently healthy canes (AC-AL), but decreased in leaves without symptoms on symptomatic canes (SC-AL). Starch grains were more abundant in AC-AL than in SC-AL containing wide plastoglobuli and numerous tannins. The starch level increased early, and then decreased with esca development. Fructose increased early by more than threefold, being the predominant soluble sugar in leaves with advanced esca symptoms. Conclusions: In response to damage induced by fungal infection, defence reactions such as fructose increased early and developed in esca leaves to scavenge reactive oxygen species and protect plant tissues. Significance of the Study: Esca infection leads to an early production of fructose involved in defence.

Foliar Symptoms Caused by Potato mop-top virus on Potato Plants During Vegetative Propagation in Scotland and Their Association With Tuber Yield, Spraing and Tuber Infection

The effect of foliar symptomatic infection by Potato mop-top virus (PMTV) on yield of tubers, spraing and infection in daughter tubers, and foliar symptom development and tuber infection in the following generations of propagation was investigated in commercial seed potato crops in Scotland. Six crops covering cvs Atlantic, Hermes, Nicola and Cara were studied between 2000 and 2006 by labelling paired replicates of plants with foliar symptoms and plants with no symptoms. Tubers from plants with no symptoms rarely produced plants with foliar symptoms in the following generation. Plants with no symptoms produced more infected tubers if they had been derived from plants with foliar symptoms the previous year than from plants with no symptoms. The proportion of daughter plants with foliar symptoms produced by tubers from plants with foliar symptoms in year 1 ranged from 19–41% and seemed to be associated with the severity of foliar symptoms. The detection of PMTV by ELISA in samples of leaves from plants with foliar symptoms ranged from 13% for cv. Cara to 59% for a crop of cv. Atlantic in 2004. The amounts of spraing were generally low but tended to be greater for tubers from plants with foliar symptoms than those from plants with no symptoms. These results indicate that roguing plants with foliar symptoms in seed potato crops could achieve an improvement in crop health but might be impractical when diseased plants are too prevalent.

Phenotypic and Etiological Differences Between Psyllid Yellows and Zebra Chip Diseases of Potato

Both potato psyllid yellows and zebra chip (ZC) potato diseases are associated with the potato psyllid, Bactericera cockerelli (Sulc). Aboveground plant symptoms of both diseases are similar but there is a difference in symptoms in potato tubers. ZC has recently been associated with a new species of the bacterium liberibacter, ‘Candidatus Liberibacter solanacearum’, also known as ‘Ca. Liberibacter psyllaurous’. Mechanisms by which the potato psyllid might cause either ZC or potato psyllid yellows symptoms are not understood. Insect transmission studies were conducted to demonstrate psyllid vectoring of both diseases and to compare symptoms and development of the two diseases. Potato plants were exposed to both liberibacter-free and liberibacter-carrying potato psyllids and later evaluated for plant and tuber symptoms. These plants and tubers were then tested for liberibacter by polymerase chain reaction (PCR). In addition, potato plants exhibiting severe psyllid yellows/ZC-like symptoms were collected from a commercial potato field heavily infested with the potato psyllid and tested for liberibacter. PCR detected ‘Ca. Liberibacter solanacearum’ in ZC symptomatic plants and tubers resulting from exposure to liberibacter-carrying psyllids. Despite development of foliar symptoms that resemble those of ZC in plants exposed to liberibacter-free psyllids, no liberibacter was detected in these plants with psyllid yellows. Moreover, tubers from these plants with psyllid yellows did not exhibit any symptoms of ZC infection and tested negative for the bacterium. No liberibacter was detected in plants or tubers collected from the psyllid-infested potato field, suggesting that the observed symptoms were due to psyllid yellows. Furthermore, potato plants that were infected with liberibacter died sooner than plants that were infected with psyllid yellows. Although an association between liberibacter and ZC has been established, no pathogen is yet associated with potato psyllid yellows and mechanisms by which psyllid yellows symptoms are induced by the potato psyllid remain unclear.

Efficacy of chemical and biological strategies for controlling the soybean brown spot (Septoria glycines)

J.D. Mantecon. 2008. Effi cacy of chemical and biological strategies for controlling the soybean brown spot (Septoria glycines). Cien. Inv. Agr. 35(2):211-214. Soybean (Glycine max) is currently the most widely grown and rapidly expanding crop in Argentina, and brown spot caused by Septoria glycines is a major foliage disease that affects this crop. Trials in the 2005 and 2006 growing seasons were undertaken using soybean (Glycine max) cv. Nidera A4800 RR. A randomized complete block design was used with four replicates. Two biological fungicides, based on Bacillus subtilis or B. pumilus, and four conventional fungicides, pyraclostrobin in combination with epoxiconazole (Opera 25 SC, BASF Argentina), azoxystrobin (Amistar 25 SC, Syngenta Argentina), trifl oxystrobin in combination with cyproconazole (Sphere 267,5 EC, Bayer, Argentina), and tebuconazole (Folicur 25 EC, Bayer, Argentina), were evaluated. Fungicides were applied at the R3 stage of soybean development. The effectiveness of the fungicide was determined by measuring disease incidence and severity. High doses of biological fungicides and all chemical treatments reduced the incidence of disease compared to the untreated control, but only Bacillus subtilis QST 713 at a high dose showed a similar effi cacy for reducing disease severity. The best results for reducing disease incidence and severity were obtained with the strobilurin fungicides pyraclostrobin plus epoxiconazole, trifl oxystrobin plus cyproconazole, and azoxystrobin. The best yields were obtained with chemical and biological treatments at high doses applied at R3; these treatments appeared to have a direct relationship with reduced development of foliar symptoms.

Quantitative Differences in Aphid Virulence and Foliar Symptom Development on Tomato Plants Carrying the Mi Resistance Gene

The Mi resistance gene in tomato reduces the feeding, fecundity, and survival of certain isolates of the potato aphid (Macrosiphum euphorbiae Thomas). This study compared the performance of two potato aphid isolates, WU11 and WU12, on nearly isogenic susceptible (Mi −) and resistant (Mi+) tomato cultivars. Although Mi significantly reduced the population growth of both aphids, WU12 numbers decreased by only 15% compared with 95% for isolate WU11. These results show that there are quantitative differences in virulence among potato aphid isolates. Compared with WU11 aphids, isolate WU12 caused more necrosis on both resistant and susceptible plants, and this increased damage may play a role in the partial virulence of isolate WU12. However, infestation with aphid isolate WU12 did not compromise plant defenses against isolate WU11 in resistant plants. Prior inoculation with either aphid isolate caused a modest reduction in the survival of WU12 adults, but this form of induced resistance was observed on both resistant and susceptible cultivars. Thus, Mi did not play a role in acquired resistance or mediate any indirect interactions between the two aphid isolates. Notably, the mode of action of Mi-mediated resistance seemed to differ depending on the aphid isolate tested. Mi dramatically deterred feeding by WU11 aphids, whereas the effects of resistance on isolate WU12 seemed to be caused primarily by antibiosis. Tolerance did not seem to be a major component of Mi-mediated responses, although resistant plants showed a modest reduction in the amount of foliar necrosis induced per aphid compared with susceptible plants.