Tamara ScullyThe powdery scab caused by the primitive organism Spongospora subterranea f.sp. subterranea can be devastating to a potato crop when conditions are right. Even when conditions are wrong, its spores remain in the soil until the right moisture, temperature and potato host are present. This is a patient microbe, whose spore balls can remain virulent for 20 years.

With the longevity of its spores in the field, this obligate parasite — now classified as being in the Protista kingdom with slime molds and Protozoa — is hard to eradicate and often goes undetected until it causes massive loss. It is widespread in North America.

Roots and tubers

Part of the confusion that once led to increasing soil populations of the disease involves two phases of the disease process. Tubers can appear disease free yet actually harbor cystosori, or spore balls, due to some potato cultivars being susceptible to root infections but showing resistance to tuber disease. These tubers are then likely to be planted for seed, leading a grower to inaccurately believe there is no disease concern in the field.

Spores found in the soil are the major infective agent of powdery scab. When the soil temperature is between 52 and 65° F, and the soil moisture levels are adequate, the spore balls release mobile zoospores. The level of soil inoculum needed to cause infection is very low. Less than one spore ball per gram of soil is more than adequate for significant disease if environmental conditions are conducive.

“Zoospores first infect the roots during the early season, producing swelling and discoloration,” Robert D. Davidson, Ph. D, Extension Specialist, Colorado State University, said. “When free moisture is present, with appropriate temperature and the host,” the spore balls release some — but not all — of their zoospores, saving some for subsequent times when conditions are favorable again. “Through the season, secondary infections can occur.”

The longer that environmental conditions remain favorable, the longer the disease process will continue to be perpetuated. Additionally, the initial zoospores will grow in the host and form secondary zoospores, which will also be released under appropriate conditions.

Decreased root mass due to infection can cause growth restriction and impact tuber growth. Root galls, which primarily form on root hairs, can continue to grow and form many spore balls. But it isn’t just the root symptoms and stages which cause problems. Tubers, too, can become infected with powdery scab.

Tubers are most susceptible to infection during the tuber initiation phase. Tubers can be infected at any time, but infections during this stage result in the most damage. Infections at later stages of tuber development may not show any signs of disease.

Tuber symptoms are scab-like lesions on the skin that are filled with a “powdery mass of spare balls,” Davidson said.

If the symptoms are mild at harvest, and the potato is stored, the lesion will continue to grow, impacting grade. And, infected tubers are likely to become dehydrated during storage.

Even without tuber symptoms, root infections can be present, and can be severe. The resulting non-observed increase of soil inoculum, is one of the reasons the disease is so prevalent and difficult to eradicate.

Environmental control

Powdery scab is a disease that is very dependent upon free moisture, needed temperatures and a susceptible host. Along with excessive water, a repeated cycle of wet and dry periods can favor disease spread.

When planting tubers, certain areas of the hill are more susceptible to remaining moist. Certain areas of flat beds also tend to hold onto moisture. Peaked hills can alleviate wet areas, but increase the risk of sun damage. The soil moisture level at 10 inches deep is greater than at six inches.

“A conducive environment around the tubers is one of the most influential aspects of the disease process,” Davidson said.

If there is an excess of moisture during tuber initiation, powdery scab infection increases. The rate of tuber infection, and the prevalence and severity of tuber symptoms also increases. If tubers have a deficit of water during the initiation phase, tuber disease symptoms are nominal, but there is risk of yield loss, Davidson said.

Spores can be carried by the wind, as well as by water, so fields where potatoes have been grown in the past two decades could be sources of inoculum for nearby fields new to potatoes. Nightshade weeds are also thought to harbor the disease agent.

Cultivar resistance

All potatoes are susceptible to powdery scab, but the degree to which they are susceptible varies greatly. And, some cultivars are resistant to root disease or to root gall formation, while others show resistance to tuber disease. Russet Burbank, Freedom Russet, Canela Russet and Rio Grande Russet have low susceptibility to both root gall development and tuber infection.

Soil PCR (polymerase chain reaction) testing can help growers deduce approximate levels of spore balls in their fields. Combining this knowledge with cultivar selection, and timing the planting of cultivars to avoid tuber inhibition during conducive temperature ranges for disease proliferation, are some tools to help manage powdery scab.

There has been anecdotal evidence that resistant cultivars can help reduce pathogen loads and disease levels in future susceptible crops.

“When resistant cultivars are planted for one to two crops…it is often possible to plant susceptible cultivars after this, which show reduced disease levels,” Davidson said.

When using compost in fields, it is important to know that spore balls can be active even in livestock manure. It is possible to contaminate a field with compost and manure containing active spores.

There are some chemical controls for powdery scab, but they are only successfully used in combination with other management tools. Scabby tubers can be treated with two percent formaldehyde, although there are some side effects.

Fluazinam, a fungicide, can be used on tubers and in furrows at planting.

“Powdery scab is a disease that can be devastating, is hard to control, and is very dependent upon environment and the use of susceptible cultivars,” Davidson said. “Producers must use all of the tools at their disposal.”

Davidson’s webinar on the disease is available here https://tinyurl.com/y9lad92n.