White rust is a major spinach disease, which was until recently restricted to North America.
Outbreaks have occurred primarily in the eastern and central parts of the United States. The first case in Europe was reported in 2012 in Crete.
Symptoms and diagnosis
The first signs indicating white rust are small chlorotic spots on the leaves. Here, white blisters soon form, appearing in concentric rings round the yellow lesions. The lesions may coalesce and become necrotic. Oospores are produced in older lesions.
Conditions for disease development
While oospores are the primary source of inoculation in many cases, airborne spores from infected crops, crop debris and spontaneously emerging plants also play an important role in the distribution within a crop and between fields. The disease is also known to be seed-borne. Certain species of Chenopodium may perhaps be weed hosts. Airborne spores (sporangia) germinate in free water and produce six to nine zoospores, which usually form a cyst and subsequently penetrate the leaf. The optimum temperature for infection is between 12 and 16 °C. At these temperatures, some infection can occur during short leaf wetness periods (three hours), but it will take far longer leaf wetness periods under less favourable conditions.
Impact and importance
This disease causes considerable damage in affected plants and calls for careful management, including crop rotation, resistant varieties and fungicide treatment. Seed treatment can offer some protection against soil inoculation and can be play a major role in preventing transfer to seed.