Powdery scab

Powdery scab is a disease of potato tubers. It is caused by the cercozoan Spongospora subterranea f. sp. subterranea and is widespread in potato growing countries. Symptoms of powdery scab include smalllesions in the early stages of the disease, progressing to raised pustules containing a powdery mass. These can eventually rupture within the tuber periderm. The powdery pustules contain resting spores that release anisokont zoospores (asexual spore with two unequal length flagella) to infect the root hairs of potatoes or tomatoes. Powdery scab is a cosmetic defect on tubers, which can result in the rejection of these potatoes. Potatoes which have been infected can be peeled to remove the infected skin and the remaining inside of the potato can be cooked and eaten. 

Powdery Scab has important implications for commercial farming. Not only does the pathogen itself cause harm, but the pathogen is also a vector for potato mop-top virus, another plant pathogen. As a result, its presence greatly threatens potato yield for farmers. The burst pustules can also act as a wound for other fungi to infect, such as Phytophthora erythroseptica and Phytophthora infestans. Thus, tubers with powdery scab can have increased incidences of other devastating diseases, including pink rot, dry rot, black dot, and late blight.Potato tubers will form powdery scab pustules that inhibit their ability to be sold. Many markets decline to buy potatoes with ugly scarring even if they are safe to eat. Research has not yet found an effective way to peel the scabs without damaging the potato. Potatoes that are rejected for sale create a large financial burden on farmers. Additionally, because soil borne inoculum can survive for years as spores, the pathogen is very difficult to eliminate once present. In Great Britain a recent Potato Council funded diagnostic project discovered that as much as 82% of fields tested positive for soil inoculum