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Y. Shi, C. Rom and J.C. Correll

Disease reactions of 11 apple genotypes (Braeburn, Empire, Gala, Granny Smith, Golden Delicious, Jonathan, Jonagold, MacIntosh, Red Delicious, Red Rome and Spartan) to 3 genetically distinct bitter rot pathogens (Colletotrichum gloeosporioides [teleomorph (T) and nonteleomorph (NT)] and C. acutatum) were examined. Fruit were surface sterilized, and then inoculated either by placing a 100 ul spore suspension (2×104 spores/ml) into wounds or spraying the inoculum onto unwounded fruit. Inoculated fruit were incubated at 26C and 100% RH. Disease reactions were quantified by measuring lesion diameter and depth in wounded fruit, or counting the number of lesions on unwounded fruit. There was a significant interaction between apple genotypes and all 3 pathogenic isolates. The T isolate was the most virulent on all genotypes. In general, cultivars with the smallest lesions in the wound test had the fewest lesions in the unwounded test. Lesion number and size were significantly lower on Granny Smith, Jonagold, Jonathan, Red Delicious and Red Rome. Of the genotypes tested, Braeburn, Gala and MacIntosh apparently were the most susceptible.

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Y. Shi, C. Rom and J.C. Correll

The susceptibility of five apple cultivars to bitter rot was examined by inoculating fruit with multiple isolates of Colletotrichum gloeosporioides and C. acutatum. Fruit were inoculated at three maturity dates in 1994. Fruit maturity was analyzed for firmness, soluble solids, and acidity. `Smoothee' and `Red Rome' were wounded-inoculated by placing 0.1 ml of inoculum (106 conidia/ml) into wounds while `Granny Smith', `Golden Delicious', and `Red Delicious' were inoculated by spraying unwounded fruit with inoculum until runoff. Free moisture was maintained on fruit for 15 h by enclosing fruit in a plastic bag. Bitter rot was quantified by counting lesion number and measuring lesion diameter. In general, more-mature fruit had a higher number and larger lesions than younger fruit. Lesion diameter was highly correlated with increased fruit soluble solids (r = 0.76) and decreased firmness (r = –0.77). The results indicate that fruit susceptibility increases as fruit ages. Differences in susceptibility were observed among apple cultivars and differences in virulence were observed among bitter rot pathogens.

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Yan Shi, C.R. Rom and J.C. Correll

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.

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Y. Shi, C.R. Rom and J.C. Correll

Bitter rot, an economically important disease of apples in the southeastern U.S., is caused by a complex of plant pathogenic fungi. Fruit infection can result in large yield losses. Control of this disease is contingent upon the effectiveness of several commonly used fungicides. Two fungal species, Colletotrichum gloeosporioides (C.g.) and C. acutatum (C.a.), cause bitter rot. Isolates of both species also show a large degree of genotypic variation. The objective of this study was to determine the effect of several fungicides: benomyl (Benlate), Captan, Mancozeb (a combination of zinc ion and manganese ethylene bisdithiocarbamate, Dithane), and zinc dimethyldithiocarbamate (Ziram). Four concentrations (0, 0.5, 1, and 2 ppm) of each fungicide were used. Fungal growth was quantified by measuring colony diameters 3 and 6 days after incubation. Among the fungicides tested, only Benlate and Ziram significantly reduced fungal growth. Average growth reductions of C. gloeosporioides with Benlate and Ziram were 87% and 29%, respectively. In contrast, average growth reductions of C.a. with Benlate and Ziram were 60% and 52%, respectively. In conclusion, Benlate was the most effective fungicide in reducing overall fungal growth, while Ziram was more effective in reducing the growth of C.a.

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L.P. Brandenberger, T.E. Morelock and J.C. Correll

Spinach germplasm (707 accessions) from collections from six countries were screened for resistance to race 4 of the downy mildew pathogen Peronospora farinosa f. sp. spinaciae; these collections contained germplasm that originated from 41 countries. The predominant species examined was Spinacia oleracea L., however, eight accessions of S. turkestanica Iljin and two accessions of S. tetrandra Stev. were also tested. About 40 seedlings of each accession were inoculated. The cultivar St. Helens was included as a susceptible control in each test. The majority of accessions tested (>98%) were susceptible to race 4. Nine accessions exhibited some resistance to race 4 (9% to 38% of the seedlings within an accession were resistant), and two accessions, CGNO 9546 and SPI 82/87, exhibited a high level of resistance (60% and 80% resistant, respectively). Resistance identified in several of the accessions in this study may be useful for breeding for race 4 resistance.

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L.A. Wasilwa, T.E. Morelock and J.C. Correll

Anthracnose of cucurbits, caused by Colletotrichum orbiculare, is composed of three races (race 1, 2, and 2B). The inheritance of race 1 in cucumber is reported to be controlled by a single recessive gene. Although the mode of inheritance to race 2 in cucumber has not been determined, it has been suggested that is quantitatively inherited. Four cucumber cultivars, H19 from two sources [the commercial seed (P1) and the breeders seed (P2)], Pixie (P3), and Marketer (P4), that were considered highly resistant, moderately resistant and highly susceptible, respectively, to race 2 were used as the parents in this study. Crosses between resistant × susceptible and resistant × moderately resistant were made. Some reciprocal crosses also were made. The F1 progeny were then evaluated for resistance to race 2 in a cotyledon assay. Disease severity was assessed 8 days after inoculation using a disease rating scale of 0–7, whereby 0 = healthy plant and 7 = 100% chlorosis or necrosis. All progeny from P1 × P1 were highly resistant (disease severity 19 < 2.5); P2 × P2 and P3 × P3 were highly moderately resistant (disease severity 2.6–4.9); and all P4 × P4 progeny were highly susceptible (disease severity > 5.0) to race 2. All F1 progeny showed a continuum of disease ratings from highly resistant to moderately resistant to race 2. The disease ratings of the F1 progeny would indicate that resistance to race 2 is controlled by multiple genes.

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L.A. Wasilwa, T.E. Morelock and J.C. Correll

Anthracnose is a destructive foliage and fruit disease of cucurbits worldwide, particularly on cucumber, watermelon, and cantaloupe. Three fungal taxa have been implicated in the cucurbit anthracnose complex [Colletotrichum orbiculare (CO), C. magna (CM), and the putative teleomorph Glomerella cingulata var. orbiculare (GC)]. In the past 7 years we have assembled a large geographically diverse collection of cucurbit isolates that have been characterized for virulence, vegetative (heterokaryon) compatibility, and mitochondrial and nuclear DNA RFLPs. All isolates that are pathogenic on cucurbit foliage are CO, belong to one of the four VCGs, and belong to a single mtDNA RFLP haplotype. Three races of CO (1, 2, and 2B) can be distinguished by their disease reactions on cucumber (`Marketer' and `H19') and watermelon (`Black Diamond' and `Charleston Gray') differentials. Race 1 (cucumber pathogen) and race 2 (watermelon pathogen) were the most common. Examination of virulence on cucurbit fruit indicates that CM and GC are more aggressive than CO, indicating that they could primarily be fruit-rot pathogens. Race 1 and 2 have been used effectively for screening disease resistance in cucumber and watermelon. Isolates of CM, GC, and Colletotrichum spp. recovered from fruit lesions were not pathogenic or were weakly virulent on cucurbit foliage and were diverse with regard to VCGs, nuDNA, and mtDNA RFLPs. However, CM and GC were more virulent on cucurbit fruit than CO.

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L.P. Brandenberger, T.E. Morelock and J.C. Correll

Field observations indicate that polygenic resistance to downy mildew (Peronospora farinosa f. sp. spinaciae) was observed during the course of a breeding program to develop polygenic resistance to white rust (Albugo occidentalis). Field studies were initiated using five cultivars and one breeding line to quantify the level of resistance to downy mildew and white rust. Separate plots were inoculated with each pathogen at a specific spore concentration and then subjected to a minimum dew period of 12h. Infection was quantified by measuring latent period, lesion number, lesion size, sporulation and percent leaf area infected.

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T.E. Morelock, J.C. Correll and L.P. Brandenberger

Downy mildew (Blue mold) is probably the most common spinach disease in most parts of the world, and it can be a problem in the mid-South. Frequently, other diseases such as white rust and fusarium cause major crop loss. The Arkansas breeding program was initiated 25 years ago to address white rust and fusarium, as well as other diseases that destroy spinach crops. Since single gene resistance is not available for most spinach diseases, it was necessary to utilize polygenic resistance to develop varieties that are resistant to most of the common spinach diseases that occur in the Arkansas River Valley of Arkansas and Oklahoma. Highly resistant genotypes have been developed by using disease nurseries and field screening, so frequent selections are made based on the reaction to 3-4 diseases.

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L.P. Brandenberger, T.E. Morelock and J.C. Correll

Field observations indicate that polygenic resistance to downy mildew (Peronospora farinosa f. sp. spinaciae) was observed during the course of a breeding program to develop polygenic resistance to white rust (Albugo occidentalis). Field studies were initiated using five cultivars and one breeding line to quantify the level of resistance to downy mildew and white rust. Separate plots were inoculated with each pathogen at a specific spore concentration and then subjected to a minimum dew period of 12h. Infection was quantified by measuring latent period, lesion number, lesion size, sporulation and percent leaf area infected.