Development of Pleospora allii on garlic debris infected by Stemphylium vesicarium

Pseudothecia of Pleospora allii developed best on garlic leaf debris infect ed by Stemphylium vesicarium incubated at low temperature (5-10 degrees C) and relative humidity (RH) close to saturation. RH of less than 96% prevent ed the formation of pseudothecia, while an incubation temperature of 15-20 degrees C led to the early degeneration of pseudothecia. Under natural cond itions, colonization by pseudothecia of unburied garlic leaf debris varied between seasons from 6.0 to 15.5 pseudothecia/mm(2), whereas lower coloniza tion levels were recorded when samples were buried. Pseudothecial maturity was reached 1-4 mo after the deposition of garlic debris on the soil surfac e and 15 days after the burial of residues. In the later case, pseudothecia degenerated with degradation of the plant debris. Ascospore release, which required rainfall or dew periods, occurred between late January and late A pril depending upon the year. A high correlation was found between pseudoth ecia maturation and four meteorological variables. Two of which, i.e. the n umber of hours with RH greater than or equal to 98% and with a mean tempera ture of 4.5-10.5 degrees C, and the accumulated rainfall, explained most va riability (adjusted R-2 = 0.82-0.98 depending upon the year). A multiple re gression equation relating the pseudothecia maturity index with these two v ariables could be used to forecast the epidemic onset of Stemphylium leaf s pots in Southern Spain. Temporal progress of pseudothecia maturation was be st fitted by a monomolecular model.