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Temperature Responses of Isolates of Macrophomina phaseolina from Different Climatic Regions of Sunflower Production in Italy. L. M. Manici, Research Plant Pathologist; De-partment of Biology and Crop Protection, Experimental Institute for Industrial Crops, Via di Corticella 133, 40129 Bologna, Italy. F. Caputo, Research Technician, and C. Cerato, Research Director, De-partment of Biology and Crop Protection, Experimental Institute for Industrial Crops, Via di Corticella 133, 40129 Bologna, Italy. Plant Dis. 79:834-838. Accepted for publication 27 April 1995. Copyright 1995 The American Phylopathological Society. DOI: 10.1094/PD-79-0834.

Sixty-four isolates of Macrophomina phaseolina from sunflower, collected in four different climatic areas of Italy (north, mideast, south, midwest), were subjected to growth rate (GR) tests at 15, 25, 30, 35, and 40C, and to chlorate resistance tests. The pathogenicity of 24 isolates was also tested on maize, sorghum, soybean, sunflower, safflower, sugar beet, kenaf, and melon seedlings. The optimum temperature for growth was 30C for 62 isolates and 35C for two isolates. Isolate GR varied considerably at all temperatures (P < 0.01) but the maximum variability between isolates occurred at 15 and 40C. Isolates from the north (colder area) grew better at lower temperatures than other isolates and also showed a good adaptability to 40C. Isolates from the midwest (Mediterranean climate) had the fastest GR at 40C but the worst GR at the lowest temperature tested. Isolates from the mideast and south, with Mediterranean climate, grew better at the optimum temperatures (30 and 35) and showed the poorest adaptability to the limit temperatures (15 and 40C). Among the isolates, 95% were chlorate tolerant, 2% were chlorate sensitive and 3% were chlorate resistant; among the chlorate tolerant isolates, 47% had feathery chlorate phenotype and 48% only showed less dense colonies than the control without potassium chlorate. The isolates were very pathogenic on soybean, moderately pathogenic on sunflower, safflower, sorghum, and melon, mildly pathogenic on sugar beet and kenaf, and not pathogenic on maize using an in vitro seedling inoculation assay. These results suggest that M. phaseolina isolates from sunflower can adapt to continental conditions such as those of northern Italy, and that the fungal population, which is chlorate tolerant and pathogenic to seedlings of various cultivated species, can be a potentially widespread pathogen on many crops.

Keyword(s): charcoal rot, chlorate sensitivity