Low Disease Pressure Research Articles

Variability in control of onion white rot by Trichoderma atroviride under different disease pressures

Pellet and solid substrate formulations combined with a soil drench formulation of Trichoderma atroviride were evaluated under varying disease pressures in field trials to determine which formulation(s) provided the best control of the soil-borne disease onion white rot. Under low (11.8%) (average Area Under the Disease Progress Curve (AUDPC): 5.94%), moderate (37.7%) (average AUDPC: 11.26%) and high (64.4%) (average AUDPC: 24.76%) disease pressure, the standard fungicide programme (procymidone coated seed plus tebuconazole foliar spray) gave statistically significant disease control (76.3%, 91.3% and 58.8%, respectively) compared with the untreated control. Under low and high disease pressure, significant control, equal to the fungicide treatment, was achieved with the T. atroviride pellet plus soil drench treatment (72.0 and 39.6%, respectively). However, no Trichoderma formulation gave significant control of onion white rot under moderate disease pressure (20.7–34.2% control). When disease progress was examined over time for each trial, it was evident that the onset of disease occurred later in the growing season at the moderate disease pressure site (3 months after sowing) compared with Trial 3 at the high disease pressure site (2 months after sowing). At the low disease pressure site, the low level of inoculum and disease incidence enabled T. atroviride to bring about disease control. These results indicated that the performance of T. atroviride may be influenced as much by the timing of disease onset as the absolute disease pressure.

Response of Local Cassava Varieties in Uganda to Cassava Mosaic Virus Disease

Cassava mosaic disease (CMD) resistant varieties have been used to manage CMD in Uganda. In spite the availability of a number of CMD resistant cultivars, several local landraces are continually grown in Uganda because they possess certain desirable traits, which may not have been fully met by the resistant varieties. Field based trials were setup to evaluate the response to CMD of some landraces that became prominent after the 1990s CMD pandemic. Experiments were located in areas where CMD is reported to have subsided namely, Mukono (Central Uganda) and Bulindi (Western Uganda). Three landraces were evaluated per site along with resistant (Nase 4) and susceptible (Bao) standards. Plots were planted with materials obtained from either diseased or healthy fields. The results showed differences in sensitivity to CMD among varieties tested, with landraces consistently having higher CMD incidences than Nase 4. The landraces tested in Mukono were more susceptible to CMD than even the Bao, while at Bulindi on the other hand, only Sibampale proved to be more resistant than Bao. There were differences in the whitefly populations among the cassava varieties and locations. High whitefly populations characterized all landraces and Nase 4. In most tested varieties, there was a higher infestation of whiteflies on healthy than diseased plants. Low spread of CMD among local landraces was observed in Bulindi as would be expected in areas where CMD has almost subsided (low pressure areas). However, in Mukono, disease spread among landraces was very rapid and high CMD incidences reaching 100% were recorded regardless of whether plots were planted with healthy or diseased cuttings suggesting it to be a CMD hot-spot. Thus, the use of clean planting materials as an effective phytosanitary method can only be employed under low disease pressure while high disease pressure areas need use of resistant varieties. Research Article

The effect of foliar fungicide application timing on the control of dry bean anthracnose

Gillard, C. L., Ranatunga, N. K. and Conner, R. L. 2012. The effect of foliar fungicide application timing on the control of dry bean anthracnose. Can. J. Plant Sci. 92: 109–118. Anthracnose caused by Colletotrichum lindemuthianum is a major disease of dry bean (Phaseolus vulgaris L.), reducing seed quality and yield. A study carried out in 2005 and 2006 at Exeter, ON, and at Morden, MB, determined that a sequential application of fungicide at the correct time is crucial for the effective management of the disease. The effect of the fungicides azoxystrobin and pyraclostrobin at four single foliar application timings at 5th trifoliolate (A), 1st flower (B), full flower (C) and 10 d after full flower (D) and at three sequential timings (A+C, B+C, and B+D) were evaluated under low and high disease pressure conditions. Data were collected on leaf vein and pod infection, plant maturity, dockage, pick, seed weight, yield and return on investment. Results were analyzed using analysis of variance (ANOVA) and contrast comparisons were carried out for various treatment combinations. Differences between the two fungicides for leaf symptoms were not apparent under low disease pressure, but occurred early in plant development under high disease pressure. Pyraclostrobin-treated plots produced a higher yield under high disease pressure and better quality seeds at both high and low disease pressure conditions. A single fungicide application at the A timing gave higher yield under low disease pressure, while timings B and C gave a higher yield under high disease pressure. A sequential application often provided greater anthracnose control and improved yield and seed quality, compared with single application timings. For the sequential application timings, the highest yields occurred at the A+C timing under low disease pressure, and at the A+C or B+C timing under high disease pressure.

Prevalence and implications of sweetpotato recovery from sweet potato virus disease in Uganda

Sweet potato virus disease (SPVD) is the most important disease of sweetpotato in the tropics. It causes yield losses of up to 98% and extinction of elite cultivars. Although there are no reports of immune cultivars, disease recovery phenomenon (a manifestation of some form of resistance) was reported in many vegetatively propagated crops including sweetpotato. A total of 1320 sweetpotato cultivars from 3 major sweetpotato growing regions in Uganda were field evaluated for SPVD severity, of which 24% showed recovery from SPVD. The nature of SPVD recovery was assessed in the screenhouse using 20 distinct cultivars after graft inoculation with SPCSV and SPFMV, the SPVD causing viruses. SPVD progress in 20 graft inoculated cultivars was significantly (P<0.05) different; Munyeera was the most resistant cultivar followed by New Kawogo and Polyster. The 3 resistant cultivars (disease score 1-2) displayed recovery from SPVD which was correlated with a reduction in SPFMV titers as observed from quantitative ELISA. Reversion was not evident since all cuttings taken from recovered vine tips later tested positive for both viruses. Cultivars with the ability to recover were common in farmers’ fields in central (36.4%) and western (33.3%) regions of Uganda, which are high SPVD pressure zones. Conversely, fewer cultivars (5.6%) from the low disease pressure eastern region displayed recovery. The very susceptible cultivars (disease score of 4-5) were not prevalent in central and western regions. These observations suggest that SPVD recovery has an influence on the cultivars grown in different SPVD pressure zones, where more susceptible cultivars are preferred in areas of low disease pressure.

A Quantitative Review of Fungicide Efficacy for Managing Downy Mildew in Cucurbits

A meta-analysis of the effect of fungicides on cucurbit downy mildew was conducted using data previously published in Fungicide & Nematicide Tests and Plant Disease Management Reports from 2000 to 2008. Standardized mean effect size (the difference in disease intensity expressed in standard deviation units between the fungicide treatment and its corresponding untreated control) was calculated for each of the 105 field studies evaluating the effects of fungicides on cucurbit downy mildew. Fixed- and random-effects meta-analyses were performed on the log-transformed standardized mean effect sizes to estimate the overall mean effect size (L+), and to determine the variability in the effect size among studies. Fungicides led to a significant (P<0.001) reduction in disease with an L+ value of 1.198, indicating that, overall, fungicides were highly effective against cucurbit downy mildew. Fixed- and random-effects meta-analyses were then conducted to determine the effects of moderator variables on L+. The fixed-effects model resulted in narrower 95% confidence intervals and generally lower estimates of moderator subgroup mean effect size (L) than the random-effects models. Fungicide efficacy was significantly (P<0.001) greater in cucumber, with L being 26.5% higher in cucumber than in other cucurbits. Year of study, number of sprays, and disease pressure in the control significantly (P<0.001) affected L. Fungicide efficacy was significantly lower during the 2004-05 season than prior to or after the 2004-05 season. Studies in which disease pressure was moderate had a significantly higher L than studies with either low or high disease pressure. Fungicide efficacy was ≥22% in studies that received 5 to 6 sprays than in studies that received 1 to 4 or >6 sprays. Fungicide products led to a significant (P<0.001) reduction in disease, although there was substantial between-study variability. The pyridinylmethyl-benzamide group of fungicides (fluopicolide) was the most effective, followed by the carbamate (propamocarb) and quinone inside inhibitors (cyazofamid) group of fungicides, while the carboxylic acid amide group (mandipropamid and dimethomorph) was the least effective. A combination of protectant and systemic fungicides resulted in a proportional increase in L compared with sole application of either protectant or systemic fungicides.

Acute toxicity of fungicide formulations to amphibians at environmentally relevant concentrations

Fungicide use on row crops is increasing dramatically due to promotion of these products for general plant health and increased yield even during low disease pressure. These fungicides are applied aerially, which may lead to amphibian exposure within fields or in nearby habitat. Exposure to three of these fungicides at environmentally relevant conditions resulted in mortality for Bufo tadpoles and juveniles for two of the fungicides. Headline®, the most commonly used on row crops, resulted in >50% mortality for juveniles at corn label application rates and 100% mortality for tadpoles at one-tenth the label rate for corn. Stratego® killed 40% of exposed tadpoles on average at the corn label rate, but only 7% of the juveniles. Overall, Quilt® appeared less toxic than the other fungicides, resulting in 7 to 10% mortality of tadpoles and 4 to 22% mortality of juveniles at all concentrations (from 0.1× to 10× label rate). These fungicides, especially Headline, may pose a major hazard for amphibians exposed as tadpoles and/or juveniles at environmentally relevant concentrations.

Maximizing Economic Returns and Minimizing Stem Rot Incidence with Optimum Plant Stands of Peanut in Nicaragua

AbstractPeanut growers in Nicaragua use high seeding rates, often greatly in excess of the 19.7 seeds/m recommended for growers in Georgia. Significant yield loss to peanut stem rot (caused by Sclerotium rolfsii) often occurs because of rapid disease spread in high density plant stands. It was hypothesized that lower seeding rates would be beneficial, and field experiments were conducted from 2005 to 2006 in Nicaragua to determine the optimum plant stands for stem rot management, peanut yield, and maximum economic returns. The cultivar Georgia Green was planted in twin rows at seven locations with final stands of 4 to 30 plants/m. Paired plots were either treated or not treated with flutolanil to control stem rot. Stem rot incidence consistently increased with higher peanut plant populations in fields with significant levels (&amp;gt;5%) of stem rot. In fields with more than 5% stem rot incidence, peanut yield and gross income adjusted for seed cost were maximized at within row populations of 12 to 13 and 11 plants/m, respectively, when treated with flutolanil to reduce stem rot. When flutolanil was not applied, yield and gross income adjusted for seed cost were maximized at 10 to 11 and 8 to 10 plants/m, respectively. In locations with low stem rot incidence (&amp;lt;5%), maximum yield and gross income adjusted for seed cost were attained at within row populations of 13 and 12 plants/m, respectively. Nicaraguan growers may maximize their economic return by utilizing seeding rates to obtain final stand counts of 8 to 11 plants/m in fields with severe stem rot and 12 plants/m with low disease pressure.

Development of effective environmentally-friendly approaches to control Alternaria blight and anthracnose diseases of Korean ginseng

In ginseng fields, Alternaria blight, caused by Alternaria panax, and anthracnose, caused by Colletotrichum gloeosporoides, have become serious problems in Korea, and control of these diseases relies mainly on intensive applications of fungicides. In an effort to develop an effective environmentally-friendly control system, we utilized preventative control approaches including rain shelter shading plates and the removal of dead plants, as well as a curative control approach, such as the application of microbial agents. In the presence of rain shelter shading plates, the occurrence of Alternaria blight and anthracnose decreased significantly compared to that seen with polyethylene shading nets. In addition, the eradication of dead ginseng plants, which harboured abundant spores of the pathogens, significantly reduced the incidence of both diseases. In fields with rain shelter shading plates and in which dead plants were eradicated, four applications of a bioformulated product containing chitinolytic bacterial strains in a simple medium containing chitin provided control similar in effect to that observed with the application of the fungicide under low disease pressure. The efficacy of the bioformulated product was decreased slightly under severe disease pressure. These findings indicate that integration of the three disease management measures might constitute a new effective and environmentally-friendly system for the control of Alternaria blight and anthracnose in Korean ginseng fields.

EFFECTIVENESS OF FLUDIOXONIL AGAINST PENICILLIUM DECAY IN CITRUS FRUIT

Fludioxonil (FDL) is a 'reduced risk' fungicide recently registered for post-harvest treatments of different horticultural crops in the USA. FDL is a broad spectrum fungicide with a mechanism of action different from other fungicides currently in use for pre- or postharvest treatments of selected fruit crops, including citrus fruits in the US. The effectiveness of postharvest 30-s-dips in aqueous mixtures of (FDL) and 20°C or imazalil (IMZ) containing 300 or 600 mg L -1 (active ingredient, a.i.) at 20°C was compared for controlling green mold caused by Penicillium digitatum. In fruit treated 24h after artificial wounding or wound-inoculating with P. digitatum (10 6 conidia ml -1 ) the effectiveness of FDL was similar or slightly inferior to IMZ, respectively, following conditions of low or high disease pressure. In experiments aimed at assessing the protective activity of FDL with fruit subjected to the fungicidal treatments 24h before inoculation with P. digitatum, the effectiveness of FDL was found to be dependent on the mode of fungal inoculation. Activity of FDL was superior in fruit wounded and inoculated by spray application compared to fruit inoculated by dipping a steel rod into the conidia suspension before puncturing. The lower performance of FDL in fruit inoculated by puncturing was ascribed to the lack of systemic properties of the fungicide. However, during storage it is unlikely that fruit become wounded: re-infections may take place mainly through old wounds caused before or at the time of fungicide treatments. In normal situations FDL can be highly effective as a curative as well as a protective agent against Penicillium decay and can successfully be used in place or in combination/rotation of/with IMZ or thiabendazole to overcome the risk of selection of resistant strains of pathogens.

Genetic map construction and QTL mapping of resistance to blackleg (Leptosphaeria maculans) disease in Australian canola (Brassica napus L.) cultivars

Genetic map construction and identification of quantitative trait loci (QTLs) for blackleg resistance were performed for four mapping populations derived from five different canola source cultivars. Three of the populations were generated from crosses between single genotypes from the blackleg-resistant cultivars Caiman, Camberra and (AV)Sapphire and the blackleg-susceptible cultivar Westar(10). The fourth population was derived from a cross between genotypes from two blackleg resistant varieties (Rainbow and (AV)Sapphire). Different types of DNA-based markers were designed and characterised from a collection of 20,000 EST sequences generated from multiple Brassica species, including a new set of 445 EST-SSR markers of high value to the international community. Multiple molecular genetic marker systems were used to construct linkage maps with locus numbers varying between 219 and 468, and coverage ranging from 1173 to 1800 cM. The proportion of polymorphic markers assigned to map locations varied from 70 to 89% across the four populations. Publicly available simple sequence repeat markers were used to assign linkage groups to reference nomenclature, and a sub-set of mapped markers were also screened on the Tapidor x Ningyou (T x N) reference population to assist this process. QTL analysis was performed based on percentage survival at low and high disease pressure sites. Multiple QTLs were identified across the four mapping populations, accounting for 13-33% of phenotypic variance (V (p)). QTL-linked marker data are suitable for implementation in breeding for disease resistance in Australian canola cultivars. However, the likelihood of shifts in pathogen race structure across different geographical locations may have implications for the long-term durability of such associations.

Variety-specific Epidemiology ofCercospora beticolaSacc. and Consequences for Threshold-based Timing of Fungicide Application in Sugar Beet

In Central Europe, fungicides to control leaf spot disease in sugar beet caused by Cercospora beticola are applied based on thresholds of disease incidence (DI, per cent of infected plants). As variety-specific fungicide application was not analyzed to date, the epidemiology of C. beticola and its effect on white sugar yield (WSY) in varieties with different susceptibility were investigated at seven sites in Germany and Austria in 2004 and 2005. All varieties reached the summary thresholds 5 ⁄ 15 ⁄ 45% DI in all environments. Fitting a logistic growth curve to DI revealed significant differences among varieties. At high disease pressure, susceptible varieties reached a considerably higher disease severity (DS, per cent of infected leaf area) at harvest and a larger area under disease progress curve (AUDPC) than resistant varieties. Fitting a logistic growth curve to DS showed an increasing differentiation among varieties with time. The growth rate estimated based on the logistic growth curve was the only variable that performed equally well in differentiating varieties under low and high disease pressure. With increasing disease pressure, varieties differed considerably in WSY, but differences between susceptible and resistant varieties were significant only in some environments. The disease-loss relation between AUDPC and relative WSY was varietyspecific. Resistant varieties had an approximately identical WSY with and without infection and compensated for negative infection effects even at higher AUDPC. Therefore, at high disease pressure, resistant varieties had a higher relative yield compared to susceptible ones. However, our results indicate that there is no need to develop variety-specific thresholds, but resistant varieties reach the established thresholds later than susceptible ones. Consequently, the time of fungicide application can be delayed in resistant varieties. This will help to reduce the use of fungicides to the bare essentials as requested for the integrated crop protection management.

DESIGNING STRATEGIES TO CONTROL GREY MOULD IN STRAWBERRY CULTIVATION USING DECISION SUPPORT SYSTEMS

Grey mould is one of the major diseases in strawberry cultivation. Fungicides to control Botrytis cinerea are applied frequently during flowering and sometimes at harvest. Reduction of pesticide use is one of the major aims of the Dutch government. Implementation of a Decision Support System (DSS) helps to achieve this goal. Pin point timing of fungicide application can possibly improve the efficacy of the treatment and reduce the number of spray applications. Predicted weather data to forecast infection risks are used by most DSS’s. However in strawberry cultivation irrigation is a daily practice. The effect of overhead irrigation on the Botrytis infection risk is unknown. This is one of the reasons that strawberry growers infrequently use DSS’s. Therefore adaptation of the model to agricultural management is necessary. Under low disease pressure DSS BoWaS controlled Botrytis fruit rot 62% better then routine applications of fungicides, with a 50% reduction of fungicide input. Adding an irrigation or a disease pressure sub-routine did not improve the model under low disease pressure. BoWaS based on disease pressure and weather resulted in better control of grey mould then the weather based BoWaS, under high disease pressure. Adding an irrigation rule did not improve the model further. Using the modified BoWaS reduced fungicide input with 36% compared to routine applications with the same efficacy.

Fusarium head blight (FHB) in Flanders: population diversity, inter-species associations and DON contamination in commercial winter wheat varieties

The presence of Fusarium spp. causing Fusarium head blight (FHB) of wheat was studied in Flanders (Belgium) in 2007 and 2008. Symptoms, deoxynivalenol content (DON), Fusarium spp. and trichothecene chemotypes were determined at seven locations on different commercial wheat varieties. Overall, significant differences in disease pressure between locations and varieties were observed within 1 year. In addition, we were able to detect consistent and significant resistance differences among the common varieties both under high disease pressure (2007) and low disease pressure (2008). The accumulation of DON was not related to the presence of F. graminearum but showed a clear correlation with rainfall during and after the period of anthesis. During the two-year survey, characterisation of 756 Fusarium samples by species-specific PCR designated F. poae and F. graminearum as the predominant species in Flanders. Furthermore, most of the ears were colonised by multiple FHB pathogens in 2007 whereas the Fusarium population was less complex in 2008. Log-linear analysis of these multiple (two- and three-way) species interactions revealed a clear correlation between F. poae and several pathogens of the FHB disease complex. Finally, chemotype analysis showed that F. culmorum and F. graminearum were respectively of the NIV chemotype and DON chemotype. 3-ADON and 15-ADON chemotypes occurred in more or less equal amounts within the F. graminearum population both in 2007 and 2008. The congruence of these results with observations throughout Europe are discussed.

Quantitative review of fungicide efficacy trials for managing soybean rust in Brazil

With the arrival of Asian soybean rust (caused by Phakopsora pachyrhizi) in the Western Hemisphere in 2001, field research to optimize chemical control of this important yield-limiting disease has proliferated. We present a meta-analytical synthesis of the results of 71 uniform fungicide trials containing 930 entries (specific fungicidal treatments) conducted in Brazil from 2003/2004 to 2006/2007. Our objectives were to: (1) quantify the overall efficacy of fungicidal treatments in reducing disease and yield loss; (2) determine to what extent fungicide efficacy depends on overall disease pressure, the number of spray applications, and the amount of disease present at the time of the first application; and (3) test for differences in efficacy among fungicide classes and specific active ingredients. Weighted median response ratios for disease severity ( R S) and yield ( R Y) were 0.413 and 1.439, respectively, indicating that, on average, fungicide treatments reduced disease by 58.7% (range: −38.9–100%) and increased yield by 43.9% (range: −21.8–458%). Response ratios were dependent on disease pressure (expressed as disease severity of the untreated check), with the greatest reduction in rust severity (i.e., lowest R S values) observed for low disease pressure and the best yield response (i.e., highest R Y values) observed for high disease pressure. In trials where both one and two application schedules were included, R S and R Y were better for entries receiving two applications than for one application. In ∼65% of entries across all trials, disease was present at the time of the first application, albeit at low levels (median = 0.20% severity). Only disease severities of up to 0.05% at the time of the first application could be tolerated without affecting R S negatively, whereas presence of any disease at the first application had a negative effect on R Y, even when disease pressure was low. In general, triazole fungicides applied alone performed better than strobilurins alone, but there was a wide range in efficacy among individual triazoles, with prothioconazole and tebuconazole performing best and fluquinconazole and difenoconazole being least effective. Combinations of strobilurins with triazoles (especially those containing cyproconazole) improved disease and yield loss control compared with either class alone. In contrast, combinations of triazoles with a benzimidazole fungicide did not improve R S or R Y compared with triazoles alone. Across fungicides, R Y and R S were correlated negatively ( r = −0.6296, P = 0.0017), indicating that treatments with better disease control also had higher yields.

The Influence of Tank Mix Additives While Making Low Volume Aerial Fungicide Applications

Abstract Multiple experiments using low volume aerial applications with nine different adjuvants in combination with fungicide were conducted to compare coverage, droplet spectra, yield, and disease control. Four locations were scattered over three states: Arkansas, Kansas, and Illinois. Four aircraft with different nozzle and boom setups were used to complete the studies. Volume median size (DV0.5) ranged from 232 μm to 364 μm. Kromekote® papers used as collectors were placed in the center 9 m of each aerial swath/treatment (20 m) at three different plant heights: top corn leaf, ear leaf, and the leaf three collars below ear leaf. In each treatment, sampling consisted of 30 papers; 10 corn plants at the three heights, with all papers at 30 cm from the main stalk. DropletScan™, utilizing a high resolution color scanner to digitize the images on the Kromekote® papers, was used to analyze each paper. Means for percent area coverage and DV0.5 were used to separate differences in treatments. Relative span was calculated as an indicator of the range in the droplet spectrum. Yields were taken from the center of each treatment. Three to five replications per treatment were completed and a SAS GLIMMIX analysis was performed. Significant differences were measured among adjuvant treatments for both droplet spectra and deposition/coverage. Mixed results are reported across all treatments and locations. A mixture of water, Headline, and Crop Oil Concentrate (Treatment 2) was consistently measured with the lowest amount of coverage, and in two of four locations had the largest DV0.5. Overall, droplet spectra were impacted by the addition of adjuvants to the tank mix. The largest droplets were not associated with the best coverage, and the trend was for the lowest coverage to be associated with the smallest droplets. Treatment 5 (NIS+Interlock) had the smallest DV0.5 in three of the test locations. A limited number of significant yield differences were found. This effect could be due to low disease pressure.

Responses of non-primed or primed seeds of ‘Marketmore 76’ cucumber ( Cucumis sativus L.) slurry coated with Trichoderma species to planting in growth media infested with Pythium aphanidermatum

Aqueous slurries of commercial preparations of Trichoderma harzianum Rifai strain KRL-AG2 G41 ( Th), T. virens Strain G-41 ( Tv), or their combination ( ThTv, at half rates each of the single application rate) were applied to ‘Marktetmore 76’ cucumber seeds ( Cucumis sativus L.) that were non-primed or primed for 3 days at 25 °C either osmotically (−2.5 MPa from 0.337 molal Ca(NO 3) 2) or osmomatrically (−1.0 MPa from 0.135 molal Ca(NO 3) 2 plus −1.5 MPa from 50% water in exfoliated grade 5 vermiculite). Slurries were applied to seeds (1 mg per seed) either before or after priming. Seeds were sown in soilless, peat-based media with or without inoculation with Pythium aphanidermatum ( Pa). Protection against damping-off caused by high pressures of Pa (16% emergence in non-coated, non-primed seeds) was increased by slurry coating Th on non-primed (76.4% emergence) or on osmotically primed seeds, with coating either before or after priming having no effect on efficacy (average 62.6% emergence). Slurry coating Th on osmomatrically primed seeds failed to increase final emergence percentage (FEP). Colony forming units per three seeds (CFU, all 10 3) was 2.8 for non-primed seeds, and 3.2 and 2.6, respectively, when osmotically and osmomatrically primed seeds were coated after priming. In a second study with lower disease pressure (58.1 FEP from non-coated, non-primed seeds), slurry coating of non-primed or osmotically primed seeds with Th, Tv or ThTv reduced percentage damping-off and increased FEP. The combination coating eliminated damping-off only in non-primed seeds, and tended to reduce percentage damping-off in osmotically or osmomatrically primed seeds compared to coating with Th or Tv alone. In a third study using only non-primed seeds, slurry coatings with mefenoxam fungicide, Th, Tv, or ThTv decreased total damping-off to 2.6%, 7.4%, 2.0%, and 0%, respectively, from the 30.1% occurring in non-coated seeds. Th, Tv or ThTv applied to growth media at the same rate as the seed coating (1 mg per seed) were generally as effective as the seed coatings, and only the ThTv growth medium application eliminated damping-off. A fourth experiment revealed that Th, Tv or ThTv remained viable on non-primed seeds for up to 4 weeks (the longest storage duration) at 21 or 4 °C, but 21 °C storage resulted in faster seed germination by week 3 and higher CFU per three seeds by week 4. In summary, coating of non-primed seeds with Th, Tv or ThTv was more effective than coating primed seeds in reducing percentage damping-off. While priming treatments generally led to faster seedling emergence and greater seedling shoot fresh weight than was achieved with non-primed seeds, only for non-primed seeds was damping-off eliminated by the ThTv seed coating or growth medium application.

Blast disease influence on agronomic and quality traits of rice varieties under Mediterranean conditions

The effects of Pyricularia oryzae, the causal agent of blast disease in rice, on plant growth, grain yield, and grain quality under Mediterranean conditions were studied in 2002 and 2003. Four rice varieties were grown under high and low disease pressure, achieved by early inoculation with the blast fungus and by natural infection, respectively. Results showed that inoculation affected the overall plant growth and yield and resulted in a reduction in plant height, number of productive tillers per plant, grain weight, and grain yield. The magnitude of the reduction was dependent on the varietals' susceptibility to the disease. Both types of the disease (leaf blast and neck blast) were negatively correlated with grain yield. It was estimated that each unit increase in leaf blast rating and neck blast resulted in 5.97 and 0.23 g plant^{-1} yield reduction, respectively. Grain quality traits were less affected by the disease treatments compared to the agronomic traits. The most pronounced effect of the disease was on total milling yield, which in some cases was reduced by inoculation up to 11%.

Grafting to Manage Soilborne Diseases in Heirloom Tomato Production

Organic heirloom tomato production is limited in the southeastern United States by foliar and soilborne diseases, thermal stress, and weathered soil structure. Heirloom cultivars command a premium market, but tolerance to disease and abiotic stress is often poor. Organic growers need research that supports the advantages of market niches afforded by heirloom tomatoes through the development of integrated systems to manage pests and reduce risks of associated crop losses or low yields. Two major soilborne diseases common in the southeast, bacterial wilt (caused by Ralstonia solanacearum) and fusarium wilt (caused by Fusarium oxysporum f.sp. lycopersici), were effectively managed using susceptible heirloom scions grafted onto resistant rootstock. In naturally infested soil, bacterial wilt incidence for nongrafted ‘German Johnson’ was 79% and 75% in 2005 and 2006, respectively. ‘German Johnson’ showed no symptoms of bacterial wilt in either year when grafted onto the resistant genotypes CRA 66 or Hawaii 7996. Fusarium wilt incidence was 46% and 50%, respectively, in nongrafted and self-grafted ‘German Johnson’ controls. When ‘Maxifort’ rootstock was grafted with ‘German Johnson’, no symptoms of fusarium wilt were seen, and plants with ‘Robusta’ rootstock had an intermediate level of disease (29%). An evaluation of commercially available rootstock was carried out in three separate experiments in diverse organic production systems to determine yield impacts with low disease pressure. ‘Maxifort’ rootstock significantly increased yield in one location (P = 0.05), but ‘Maxifort’ and ‘Robusta’ rootstock did not consistently impact yield at the other two locations. Grafting is an effective management tool for organic growers in the southeast United States to reduce risk of crop loss resulting from soilborne diseases and will be a valuable component in an integrated pest management program.

EFFECT OF BLAST DISEASE ON NITROGEN ACCUMULATION AND REMOBILIZATION TO RICE GRAIN

The interaction between mineral nutrients and foliar diseases is an important factor affecting grain yield and quality of cereal crops. This study was conducted to investigate the effect of blast disease (Pyricularia oryzae Cav.) on nitrogen accumulation and remobilization to grain of rice plants (Oryza sativa L.), and to assess the possible nitrogen losses occurring between anthesis and maturity in response to disease stress in the field. Four rice cultivars with various levels of susceptibility to blast disease were grown in 2003 and 2004 under high and low disease pressure, achieved by early inoculation with the blast fungus and by natural infection, respectively. Inoculation affected the overall nitrogen metabolism of rice plants and in most cases the effect was proportional to disease pressure. High disease pressure increased the tissue nitrogen concentration and the total nitrogen content, but decreased the proportion of aboveground nitrogen that was partitioned to the grain. Post anthesis nitrogen accumulation was lower in inoculated plants, and therefore, their grain filling depended more on the existing nitrogen reserves stored in vegetative parts compared to the non-inoculated plants. Nitrogen translocation was higher in inoculated than non-inoculated plants, mainly due to the greater amount of translocation available nitrogen rather than to the higher efficiency of nitrogen translocation. The great amount of nitrogen accumulated until anthesis by the inoculated plants, in combination with the low grain demands for nitrogen, due to their lower grain yield, resulted in higher nitrogen losses between anthesis and maturity.

Nitrogen utilisation efficiency and grain yield components of rice varieties grown under blast disease stress

The aim of this study was to analyse the relative importance of traits causing variation in grain yield and nitrogen utilisation efficiency (NUE) among rice (Oryza sativa L.) varieties grown under blast disease (Pyricularia oryzae Cav.). Four rice varieties with various levels of susceptibility to blast disease were grown in field experiments in 2003 and 2004. Plants were grown under high and low disease pressure, achieved by early inoculation with the blast fungus and by natural infection, respectively. Inoculation with the fungus reduced the grain weight compared with naturally infected plants. The magnitude of this reduction was proportional to disease severity. Grain nitrogen (N) concentration was higher in the inoculated plants. Increased grain N concentration resulted in a reduction of its contribution towards grain N yield variation. Inoculation reduced the NUE for biomass production (NUEb) and the NUE for grain yield (NUEg). NUEb was negatively correlated with leaf blast rating, suggesting that the higher disease pressure was on leaves, the lower the biomass productionwas perNunit.Nharvest index accounted for almost all the variation in NUEg among varieties in the inoculated plants. Results indicated that the higher dilution of N by dry matter in the non-inoculated plants was responsible for the higher NUEg compared with the inoculated plants.