Differential Control Efficacies of Vitamin Treatments against Bacterial Wilt and Grey Mould Diseases in Tomato Plants.

Bacterial wilt and grey mould in tomato plants are economically destructive bacterial and fungal diseases caused by Ralstonia solanacearum and Botrytis cinerea, respectively. Various approaches including chemical and biological controls have been attempted to arrest the tomato diseases so far. In this study, in vitro growths of bacterial R. solanacearum and fungal B. cinerea were evaluated using four different vitamins including thiamine (vitamin B1), niacin (vitamin B3), pyridoxine (vitamin B6), and menadione (vitamin K3). In planta efficacies of the four vitamin treatments on tomato protection against both diseases were also demonstrated. All four vitamins showed different in vitro antibacterial activities against R. solanacearum in dose-dependent manners. However, treatment with 2 mM thiamine was only effective in reducing bacterial wilt of detached tomato leaves without phytotoxicity under lower disease pressure (106 colony-forming unit [cfu]/ml). Treatment with the vitamins also differentially reduced in vitro conidial germination and mycelial growth of B. cinerea. The four vitamins slightly reduced the conidial germination, and thiamine, pyridoxine and menadione inhibited the mycelial growth of B. cinerea. Menadione began to drastically suppress the conidial germination and mycelial growth by 5 and 0.5 mM, respectively. Grey mould symptoms on the inoculated tomato leaves were significantly reduced by pyridoxine and menadione pretreatments one day prior to the fungal challenge inoculation. These findings suggest that disease-specific vitamin treatment will be integrated for eco-friendly management of tomato bacterial wilt and grey mould.

Evidence for Genetic Similarity of Vegetative Compatibility Groupings in Sclerotinia homoeocarpa.

Vegetative compatibility groups (VCGs) are determined for many fungi to test for the ability of fungal isolates to undergo heterokaryon formation. In several fungal plant pathogens, isolates belonging to a VCG have been shown to share significantly higher genetic similarity than those of different VCGs. In this study we sought to examine the relationship between VCG and genetic similarity of an important cool season turfgrass pathogen, Sclerotinia homoeocarpa. Twenty-two S. homoeocarpa isolates from the Midwest and Eastern US, which were previously characterized in several studies, were all evaluated for VCG using an improved nit mutant assay. These isolates were also genotyped using 19 microsatellites developed from partial genome sequence of S. homoeocarpa. Additionally, partial sequences of mitochondrial genes cytochrome oxidase II and mitochondrial small subunit (mtSSU) rRNA, and the atp6-rns intergenic spacer, were generated for isolates from each nit mutant VCG to determine if mitochondrial haplotypes differed among VCGs. Of the 22 isolates screened, 15 were amenable to the nit mutant VCG assay and were grouped into six VCGs. The 19 microsatellites gave 57 alleles for this set. Unweighted pair group methods with arithmetic mean (UPGMA) tree of binary microsatellite data were used to produce a dendrogram of the isolate genotypes based on microsatellite alleles, which showed high genetic similarity of nit mutant VCGs. Analysis of molecular variance of microsatellite data demonstrates that the current nit mutant VCGs explain the microsatellite genotypic variation among isolates better than the previous nit mutant VCGs or the conventionally determined VCGs. Mitochondrial sequences were identical among all isolates, suggesting that this marker type may not be informative for US populations of S. homoeocarpa.

Rapid development of fungicide resistance by Sclerotinia homoeocarpa on turfgrass.

Dollar spot, caused by Sclerotinia homoeocarpa, is the most prevalent and economically important turfgrass disease in North America. Increasing levels of fungicide resistance, coupled with tightening environmental scrutiny of existing fungicides, has left fewer options for managing dollar spot. More knowledge about S. homoeocarpa populations is needed to improve dollar spot management strategies, especially with respect to minimizing the development of fungicide resistance. Population diversity of S. homoeocarpa was examined using inter-simple sequence repeat markers and vegetative compatibility assays. Two subgroups were found in S. homoeocarpa field populations on both fairway and putting green turfgrass at a research field in Wisconsin. These subgroups were genetically different, vegetatively incompatible, and had different fungicide sensitivities. The frequency of the two genetic subgroups differed significantly between the fairway and putting green, but was uniform within the fairway or within the green. Population dynamics of S. homoeocarpa in response to two systemic fungicides (thiophanate-methyl and propiconazole) were assessed based on in vitro fungicide sensitivity. Dynamics of S. homoeocarpa populations depended on the presence of fungicide-resistant isolates in the initial populations before fungicide applications and changed rapidly after fungicide applications. Shifting of the population toward propiconazole resistance was gradual, whereas thiophanate-methyl resistance developed rapidly in the population. In conclusion, field populations of S. homoeocarpa containing genetically distinct, vegetatively incompatible groups were different on turfgrass that was managed differently, and they were changed rapidly after exposure to fungicides.

Evaluation of Virulence to Adlays of Korean Isolates of Bipolaris coicis Using a Disease Rating Scale.

The virulence of 33 isolates of Bipolaris coicis from diverse geographic origins in Korea was evaluated on six adlay cultivars or lines under controlled environmental conditions. To evaluate infection levels of B. coicis isolates in adlay plants, a 10-class disease rating scale was developed based on the qualitative and quantitative differences of lesions on adlay leaves infected. There were significant differences in virulence among isolates of B. coicis, although the differences were quantitative rather than qualitative. Significant differences also were observed among cultivar-isolate interactions in the analysis of variance. All isolates of B. coicis, except KG-9515, were pathogenic on adlay plants. BC-20136, the most virulent isolate, was capable of causing highly susceptible reactions on the adlay cultivars or lines. Significant differences in levels of resistance were found among six adlay cultivars or lines tested. The disease rating scale may be a reliable criterion to evaluate a large number of adlays for resistance to leaf blight or B. coicis isolates for virulence to adlays.

Effects of Plant Age, Leaf Position, Inoculum Density, and Wetness Period on Bipolaris coicis Infection in Adlays of Differing Resistance.

The effects of plant age, leaf position, inoculum density, and wetness period on the development of adlay leaf blight caused by Bipolaris coicis were evaluated using six adlay cultivars or lines of differing resistance in field plots and in the greenhouse. Levels of resistance to leaf blight among the six cultivars or lines were consistent between the 1998 and 1999 field experiments. With the aging of adlay plants, leaf blight development gradually increased in all cultivars or lines tested. Leaf blight severity was significantly greater on the lower (older) leaves than on the upper (younger) leaves. As inoculum density increased from 103 to 106 conidia per ml, the development of leaf blight increased. Disease severity also increased as the time of leaf wetness duration increased from 0 to 60 h. Wetness duration above 48 h and a high inoculum density at 106 conidia per ml caused severe leaf blight symptoms in adlay seedlings. The data from the greenhouse experiments consistently revealed the same trends in relative levels of susceptibility of adlay cultivars to leaf blight as those seen at late plant growth stages under field conditions.

Relationship of Host Genotype to Bipolaris Leaf Blight Severities and Yield Components of Adlay.

Leaf blight, caused by Bipolaris coicis, is one of the most destructive and major yield-limiting diseases of adlay (Coix lacryma-jobi var. ma-yuen). Adlay is used as an ornamental, and as a medicinal and cereal food in many countries. Field studies performed from 1997 to 1999 in Korea evaluated the resistance of 30 adlay genotypes to Bipolaris coicis and determined the relationship of cultivar resistance to leaf blight development and yield. Adlay response to leaf blight was quantitative rather than qualitative, varying greatly among cultivars or lines tested. Disease severity was lowest in Tsukuba B7, Tsukuba B15, and Muzu, and highest in Huksuk 1, Limgae, and Huksuk 2. Leaf position significantly affected disease severity, with lower leaves being more severely infected than upper leaves. Yield reductions of up to 52.4% for line Huksuk 2 were observed and were primarily associated with reductions in kernel numbers per plant, thousand kernel weight, and percent kernel ripeness. Loss of yield and yield components by leaf blight were less in resistant line Tsukuba B15 than in susceptible line Huksuk 2.