06-13-18 CO Potato Disease Alert: Soil Testing for Powdery Scab at San Luis Valley Research Center

Soil Testing for Powdery Scab at San Luis Valley Research Center

In the San Luis Valley, powdery scab disease is developing into a major concern for potato production in recent years. Spongospora subterranean is a soil borne pathogen that can infect root hairs, and at early stages of epidermal cells of a stolon. Infected tubers show lesions on the tuber skin, and roots develop root galls. In severe cases tubers with the disease are highly prone to weight loss in storage, and more vulnerable to secondary infection. Powdery scab is also responsible for the transmission of Potato mop-top virus (PMTV) which causes significant suppression of plant growth and internal tuber necrosis.

Powdery scab symptoms can cause significant economic losses in both fresh and seed markets due to the rejection of shipments. Potato cultivars with smooth or light skin (i.e., whites and reds) are more susceptible; whereas russet-skinned cultivars are less prone to powdery scab. The pathogen survives in soil or potato debris. Inoculum levels in the soils have risen to high levels due to susceptible cultivars, not having nonhost rotation intervals, non-availability of seed treatments.

So far there is no fungicide application that can completely control powdery scab pathogen on the seed tuber. Powdery scab control with soil chemical treatments may not be environmentally friendly and effective. There have been many efforts by different potato breeding programs to develop new durable resistance varieties by integrating wild germplasm. To date, no resistant cultivars have been developed to reduce losses. Recent studies on biocontrol agents such as Trichoderma spp. have shown activity on resting spores, or zoospore activity and infection. More work is needed to fully deploy these methods to control powdery scab in the field.

The control strategies suggested to reduce the incidence of powdery scab disease include field choice and crop rotation, pathogen detection, planting disease- and pathogen-free seed tubers. No single method is likely to give complete control of powdery scab.

Diagnostic Testing: Soil samples testing service is provided to identify fields that are infected with powdery scab. Information on the spore ball count for per gram of the soil will be provided so that growers can plan which variety to plant in the next season. At San Luis Valley Research Center, Postharvest Program also provides similar services on Black dot and Silver scurf.

Contact numbers are 719-480-9042/719-754-3594 x11.