Powdery Mildew of Tomato
Tomato powdery mildew may be caused by three pathogens worldwide.
Leveillula taurica ( Oidiopsis taurica) is a pathogen of a wide range of host species in warm arid to semiarid climates in Asia, the Mediterranean, Africa, and more recently the southwest United States. Erysiphe orontii ( E. cichoracearum and E. polyphaga) is another species common to many host plants in both temperate and tropical regions. And as a third species Odium lycopersicum.
Factors for Disease Development:
x) relative humidity levels > 50% (optimum RH > 90%)
x) free water on leaf surfaces is not necessary
x) temperature range: 10-35 °C (best below 30 °C)
Powdery mildew is an inoculum driven disease. Therefore just risky periodes could be determined, main factor for the damage, e.g. the outbreak is the initial inoculum (which is active in a broad range of temperature). So for control stragies: combine the modeling of the risky periode together with the monitoring of fungal inoculum (disease) in the field!