Relationships between Weather Variables, Airborne Spore Concentrations and Severity of Leaf Blight of Garlic Caused by Stemphylium vesicarium in Spain

Ascospores and conidia released into the air were recorded around plots on which garlic debris infected by Stemphylium vesicarium were fixed onto the soil surface. Symptoms in garlic trap plots located in the vicinity of infected debris, started in March and developed during April–May to reach disease incidence close to 100%, final disease severity values being lower in 1993 and 1995 than in 1994 and 1996. Whereas daily concentrations of ascospores were rather erratic, with 30% of captures between 0 and 6 h, conidia showed a daily periodicity with highest concentrations between 12 and 18 h, with a pronounced peak between 14 and 16 h, and lowest values at night. Ascospore release occurred mainly in February and March. It coincided with rainfall periods, 14 h with vapour pressure deficit 5 mb and solar radiation <145 W m^–2 on the current day of the capture. In contrast, greatest captures of conidia started in late April and were prevalent in May, and were associated with rainfall in days previous to the capture in which rather high temperature occurred and solar radiation was 109–345 W m^–2. Among the weather variables considered, rainfall appeared directly related to the aerial concentration of ascospores and conidia. The role of relative humidity seemed essential when rainfall did not occur. There was a relationship between conidia concentration in the air and number of hours with temperature in the range 12–21 °C. Ascospore production was not essential for infections to take place, since primary infection from conidia may occur and disease can develop from them readily.