Variability in the Italian population of Gnomonia leptostyla, homothallism and resistance of Juglans species to anthracnose

Isolations of Gnomonia leptostyla were carried out in 13 plantations of Juglans regia distributed throughout Italy, including Sardinia and Sicily. Mean diameter of colonies grown in vitro at 22 degrees C and sporulation of 191 isolates, grouped by site of collection, were compared. Four groups of isolates, from environmentally diverse sites, were more thoroughly investigated for the effect of temperature (from 10 to 30 degrees C, 5 degrees C increment) on growth and sporulation. Colony growth of isolates within groups correlated with temperature and environmental parameters at the site of collection, such as temperature, rainfall, number of rain days per month and per year, and altitude. Altitude and mean temperature in April correlated with colony growth. In general, the isolates that grew significantly more slowly were from sites with colder early springs and higher altitudes. More significant data were obtained comparing isolates from different sites obtained from the same walnut cultivar or population. The host source was more significantly correlated with colony growth within the same site of collection. Acervular conidiomata were abundantly produced by all isolates at 22 degrees C in darkness after 21 days, while protoperithecia were produced within 2 months by most isolates, under the same conditions. Similarly, the four groups of isolates tested at different temperatures produced conidiomata at 20 and 25 degrees C, but not at 10, 15 and 30 degrees C. After 2 months, protoperithecia were present in most isolates at 20 degrees C, very few at 25 and 15 degrees C, and no production was recorded at 10 and 30 degrees C. Fertile perithecia with asci and ascospores were produced, after 3 months at 10 degrees C in darkness, by six homothallic isolates out of 38 tested. In general, perithecia were larger than protoperithecia both in ascocarp diameter and in neck length. Ascocarp diameter, width of asci and length of ascopores of in vitro-produced perithecia were larger than those of perithecia produced in nature. The latter showed a neck length longer than in vitro-produced perithecia. Virulence of isolates when inoculated on J. regia was correlated with colony growth rate, but rated independently for homothallic or heterothallic isolates. A preliminary screening for sources of resistance to anthracnose in the genus Juglans showed J. sieboldiana and J. cinerea to be highly resistant and both J. nigra and J. hindsii to be highly susceptible to the disease. Juglans regia showed an intermediate response of susceptibility to anthracnose.