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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.
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.
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.