Factors Affecting Growth, Sporulation, Pathogenicity, and Survival of Ascochyta Rabiei

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Abstract
Isolates of Ascochyta rabici from diseased chickpea (Cicer arietinum) from India, Iran, Turkey, and West Pakistan varied greatly in growth rate, sporulation, and colony appearance. Maximum spore production on various nutrient media occurred with 8% chickpea seed meal agar (CSMA), whereas mycelial growth was usually greatest on CSMA or oatmeal agar. The optimum temperature for growth and sporulation was 15–20 C and the upper limit for growth was 30 C. Exposure of uniform colonies to continuous light on potato-dextrose agar increased mycelial growth rates and conidial production, whereas zonation occurred in most isolates exposed to alternating light and dark periods. Wavelengths of light in the near-ultraviolet and blue regions of the electromagnetic spectrum were responsible for increased growth and sporulation. In inoculation studies, A. rabiei infected chickpea, Phaseolus vulgaris, and Vigna sinensis, but was highly pathogenic only to chickpea. Of numerous chickpea lines tested, only one black-seeded type was highly resistant to most isolates of the pathogen. Pycnidia containing viable spores formed on dried chickpea stem pieces at temperatures of 10–30 C, but developed most quickly at 20 C. Mature pycnidia formed at 20 C in 46 hr in continuous light, 50 hr in alternating light and dark, and 68 hr in continuous dark. The fungus survived over 2 yr in naturally-infected chickpea tissue at temperatures of 10–35 C, relative humidities of 0–30% and on the soil surface. Ascochyta lost its viability rapidly at 65–100% relative humidity and at soil depths of 10–40 cm.