Powdery mildew of Onions

Leveillula taurica, Oidiopsis sicula

Hosts and Symptoms
L. taurica is the pathogen responsible for powdery mildew on onions, but it can also infect peppers, tomatoes, eggplant, cotton, and garlic. While L. taurica can infect many different plants it is actually very host specific. Different races of L. taurica can only infect certain crops, and even specific cultivars within the same crop. An accurate way to describe its host specificity is that this disease is, “a composite species consisting of many host-specific races." 
Symptoms of Onion Powdery Mildew (OPM) are usually seen as circular or oblong lesions that are 5 to 20 mm and have a chlorotic or necrotic appearance. The lesions appear on older leaves before the bulb of the onion begins to form, but also can occur on the younger leaves towards the end of the season. 
As the disease progresses signs of OPM can also be seen. On the lesions white mycelium can be found with conidiophores bearing either lanceolate or rounded condia.

Disease Cycle
The polycyclic disease cycle of L. taurica is similar to that of other powdery mildew species. It overwinters (as chasmothecia) in crop residues above the soil surface. Under favorable climatic conditions, the chasmothecia open and release ascospores, which are wind-dispersed. The ascospores enter the host through its stomata, germinate, and colonize the host’s tissues with its mycelia. The pathogen then begins to produce its asexual conidia, either singly or on branched conidiophores. The conidia exit through the host’s stomata and serve as a secondary inoculum to spread disease after initial infection. In the fall, the pathogen undergoes sexual reproduction and again produces chasmothecia, its dormant, overwintering structure.

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