Abstract
Six strains of bacterial canker (Clavibacter michiganensis ssp. michiganensis), isolated from tomato plants from greenhouses or fields near Cleveland, Ohio, were tested for virulence. The most virulent of the strains was used to evaluate 13 cultivars reported to be resistant to bacterial canker. Eleven of the cultivars were resistant at a high inoculum level (8.5 × 108 cells per plant) and the other two only at a lower level (8.5 × 102 cells per plant). It was demonstrated that it is possible to identify plants with intermediate resistance using a dilute inoculum of a virulent strain of C. michiganensis
populations has been reported as an allele fixation mechanism that favors rapid adaptation to new hosts and the emergence of new genotypes ( Lamour et al., 2012a ). The control of P. capsici is complex because of the presence of different virulence
cultivars and varieties to P . capsici has been difficult due to the presence of diverse virulence phenotypes of this pathogen ( Barchenger et al., 2018b ; Jiang et al., 2015 ; Oelke et al., 2003 ; Sy et al., 2008 ). Virulence phenotype
., 2002 ). The deletion of bcsod1 in the necrotrophic fungal pathogen Botrytis cinerea causes smaller lesion on pea ( Pisum sativum ) leaves, which proves that this gene is essential for full virulence of B. cinerea on plants ( Rolke et al., 2004
Several strains of Agrobacterium tumefaciens and A. rhizogenes were shown to form tumors on runners of the diploid strawberry species Fragaria vesca L. Tumors, weighing from 0.1 to 8.3 mg, appeared from 2 to 4.5 weeks after infection. The majority of tumors tested for opine synthesis by high-voltage paper electrophoresis analysis showed positive results. These results demonstrate that diploid strawberry plants are susceptible to infection with Agrobacterium and that there are differences in the relative virulence of Agrobacterium strains.
Three taxa, Colletotrichum orbiculare, the unconfirmed teleomorph of C. orbiculare (Glomerella cingulata var. orbiculare), and C. magna, have been reported to cause anthracnose of cucurbits. In a previous study, virulence, vegetative compatibility, and mtDNA RFLPs have been used to examine these taxa. The three taxa can be distinguished based on mtDNA RFLPs. Under controlled greenhouse inoculation tests, only isolates of C. orbiculare (CO) from cucurbit hosts were highly virulent on cucurbit foliage; isolates of G. cingulata (GC) and C. magna (CM), and CO from cocklebur hosts were weakly virulent or avirulent. The majority of CM and GC isolates were recovered from fruit, whereas most CO isolates were recovered from foliage. A study was conducted to evaluate the pathogenicity and virulence of anthracnose isolates on cucurbit fruit. Twenty-seven isolates of the three taxa were selected based on the host and geographic origin, mtDNA RFLP haplotype, vegetative compatibility group, and race. Mature fruit from cucumber cultivars Marketer (susceptible) and H19 (resistant) and watermelon cultivars Black Diamond (susceptible) and Charleston Gray (resistant) were used. Fruit were inoculated by placing Torula yeast agar inoculum plugs (8mm in diameter) into wounds. Following inoculation, the wounds were covered with Parafilm and incubated for 8 days at 25C at 100% RH. On the third day the Parafilm was removed from the wound. Disease symptoms were evaluated by measuring lesion diameter and depth and evaluating the presence or absence of sporulation. All three anthracnose taxa are capable of infecting cucurbit fruit. CM and GC isolates were more virulent than CO isolates on cucumber. In contrast, on `Black Diamond', CO isolates were more virulent than CM and GC isolates. No significant differences in virulence were observed on `Charleston Gray'. There were no significant differences in virulence between the races of CO except on `Charleston Gray', where race 2 isolates were significantly more virulent than race 1. CO isolates from cocklebur were only weakly virulent on cucurbit fruit.
The virulence of six strains of Erwinia amylovora used in combination for screening fire blight resistance of pear seedlings and advanced selections from the Harrow pear breeding program was evaluated by inoculating a standardized suspension (108 cfu/ml) of the six strains individually and in combination into actively growing shoot tips and measuring the lengths of the diseased shoots six weeks later. Three cultivars provided a range of resistance to fire blight: `Bartlett' was susceptible, HW-605 (`Seckel' × NJ-6) was moderately resistant, while `Kieffer' was resistant. On `Bartlett', one strain was consistently more virulent than the combination, while on HW-605, two strains were consistently more virulent than the combination. One strain was consistently less virulent than the combination on both `Bartlett' and HW-605. No strain was consistently more or less virulent than the combination when inoculated into `Kieffer'. Lesion lengths were greater in the susceptible cultivar `Bartlett' than in either HW-605 or `Kieffer'. These results suggest that a combination of strains of E. amylovora is appropriate for screening for fire blight resistance in pear genotypes.
Apple bitter rot, caused by Glomerella cingulata is an economically important disease in the Southeastern U.S. Development of resistant cultivars may be an important way to control this disease. To evaluate the apple germplasm resistance, it is necessary to understand variation in virulence of isolates so that appropriated isolates can be selected for screening procedures. Examination of virulence was performed on four Arkansas isolates (G667, G668, G959 and G960) on `Golden Delicious' fruit. The fruit were wound-inoculated with a 100 μl spore suspension (107 spores/ml), then incubated in dew chamber at 28C and 100% RI-I. Differences in virulence were detected among the isolates in terms of lesion diameter and depth. Isolate G959 was statistically (p=.05) more virulent than two isolates (G667 and G668) examined. Additional isolates from difference geographical locations will be examined for virulence in future studies.
Virulence of morphologically diverse isolates of Glomerella cingulata (anamorph: Colletotrichum gloeosporioides) and Collectotrichum sp. was examined by inoculating apple fruit. Three morphologically distinct fungal pathogens were examined on Red Delicious, Golden Delicious, or Idared. Fruit were inoculated by either placing a 100 ul spore suspension (106 spores/ml) into wounds or spraying the inoculum onto wounded fruit. All fruit were incubated at 25C on 100% RH. Free moisture was maintained on spray inoculated fruit. Virulence was quantified by measuring both lesion diameter and depth every 2-5 days for 2-4 weeks after inoculation. Overall, all of the teleomorphic isolates (G. cingulata) were significantly (p=.05) more virulent than the nonchromogenic or chromogenic isolates on fruit of all three cultivars. Genetically and morphologically diverse isolates of the bitter rot pathogen(s) are being selected and used to evaluate cultivar resistance to fruit rot.
genotype. Over the years, multiple research groups have collected isolates within their geographic region and classified them into physiological races based on virulence on a set of independently chosen host cultivars ( Bolton and Svejda, 1979 ; Debener et