MSU Extension

Potato late blight has been found in a few more fields in Montcalm County and two fields in St. Joseph County in southern Michigan in 2015. Some crops in southern Michigan are now in early decline and ready to be desiccated or already being harvested. The amount of disease is generally light in southern counties. Areas in fields that were vulnerable were field margins, especially those close to tree lines, raised cable lines and where water accumulated, such as around pivot tracks and tractor wheel lines. The genotype of the Phytophthora infestans isolate responsible for all late blight confirmations in potatoes have been the US-23 genotype. This was confirmed by GPI allozyme analysis.

Conditions remain conducive for late blight in potato crops and the risk of tuber blight is high, especially in fields in areas that have experienced heavy rain over the past few weeks. Forecasts and disease severity values (DSV) accumulations can be checked daily at the Michigan Late Blight Risk Monitoring website and will be available up to harvest this year. Growers should sample symptomatic tubers and send them for diagnosis to the Kirk lab, 612 Wilson Rd. Room 35, East Lansing, MI 48824.

The indication of high risk was communicated early in the season to the industry. This included information from Michigan State University that winter temperatures were likely to have resulted in a high likelihood that volunteer tubers would have survived the winter, and feedback from scouts that survival of volunteer potatoes was profuse. This intense early scouting resulted in late blight being reported early and the flurry of early protection, which may have resulted in many acres of potatoes being spared from late blight in 2015.

Late-season disease biology and recommendations

Sporulation in this pathogen is favored by wet weather with moderate temperatures of 60-80 degrees Fahrenheit, high humidity and frequent rainfall. Under such conditions, the disease can spread extremely rapidly and has the potential to completely defoliate fields within three weeks of the first visible infections if no control measures are taken. In addition to attacking foliage,P. infestans can infect tubers at any stage of development before or after harvest and rot of tubers often occurs in storage following tuber infections. Late blight-infected tubers have been found already in Michigan and again the genotype has been US-23.

Symptoms

Late blight infection of tubers is characterized by irregularly shaped, slightly depressed brown-to-purplish areas on the skin. These symptoms may be less obvious on russet and red-skinned cultivars. A tan to reddish-brown, dry, granular rot is found under the skin in the discolored area, extending into the tuber usually less than half an inch. (See Michigan State University Extension bulletin E2945, “Michigan Potato Diseases: Late Blight.”) The extent of rotting in a tuber depends on the susceptibility of the cultivar, temperature and length of time after the initial infection. The margin of diseased tissue is not distinct and is marked by brown, finger-like extensions into the healthy tissue of the tuber. In time, the entire tuber becomes blighted and discolored.

Late blight rot of tubers is often accompanied by soft rot and subsequent breakdown in the storage. Positive identification of late blight can be made by microscopic examination of lesions from tubers collected when the pathogen is producing spores. This water mold can be quickly identified by the distinctive size and shape of the spores and spore bearing stalks.

Late-season disease cycle

Sporangia may germinate at temperatures between 44 and 55 F, which can occur during September, when free water is present for eight to 12 zoospores per sporangium. These swim freely in water films, attach to the tuber (encyst) and infect the tuber. At temperatures of 55 to 70 F, sporangia germinate by means of a single germ tube. Nighttime temperatures of 50 to 60 F accompanied by light rain, fog or heavy dew, followed by days of 60 to 75 F with high relative humidity, are ideal for tuber late blight infection and development. Tubers may become infected if sporangia produced on the foliage are washed down into the soil by rain or irrigation water. Water-borne spores appear to follow stems and stolons in a water film into the soil, reach tubers and cause infection. Tubers near the soil surface are thus more likely to be infected.

Phytophthora infestans can only survive in living potato tissue, and usually survive from year to year in infected tubers placed in storage, piles of cull potatoes or infected tubers missed during harvest that remain unfrozen over winter (volunteer potatoes). It is therefore important to harvest and remove as many tubers as possible from the field.

In spring 2016, the pathogen can be transmitted from infected tubers in cull piles or volunteers in fields from the previous season to potato foliage by airborne spores. Infected seed potatoes are also an important source of disease. Infected tubers may rot in the soil before emergence, and not every potato that emerges from an infected tuber will contract late blight. Phytophthora infestans sporangia may be spread from infected plants in one field to healthy plants in surrounding fields by wind, splashed rain, mechanical transport and animals, thereby continuing the disease cycle. Many reproductive cycles are possible within a season that accounts for the rapid increase in disease once it becomes established in a field.

Recommendations

Late in the season, it is advisable to avoid excessive irrigation as tubers become infected with late blight when spores wash down through the soil from infected leaves. Late-season fertilizer applications should also be limited as although they will maintain green vines and promote tuber bulking, green and vigorous vines can also be difficult to kill with desiccants, and immature tubers are more prone to skinning and therefore infection at harvest. Green vines may also harbor inoculum that can infect tubers during harvest.

At the end of the season, petiole nitrate levels should drop down to levels that encourage vine senescence. Vines should also be killed at least two weeks before harvest, especially in blight-infected fields. This interval minimizes the chance of tubers getting contaminated with late blight inoculum during harvest, and allows previously infected tubers to decompose in the field. If blight is present in the field or in the vicinity of the field at harvest, it may also be beneficial to spray foliage after vine killing with labeled fungicides to kill living late blight spores on the foliage. Oxidate, copper products or Super Tin (Agri Tin) could be used, but for the latter two, growers must pay attention to maximum seasonal rates of 11.25 ounces per acre per season. See the Fungicide Rates section of the Michigan Late Blight Risk Monitoring website for more information on fungicide seasonal rates, and see MSU Extension bulletin E0312, “Insect, Disease and Nematode Control for Commercial Vegetables,” for recommendations on storage products.

Finally, after harvest if tubers are stored, they should be dry when placed in storage, and the storage air temperature and humidity should be managed so that the tubers remain dry. Condensation of moisture on tubers, resulting from air circulating through the tubers that is warmer than the temperature of the tubers, will cause any late blight present to form spores, and late blight may spread in the pile. Potatoes should be held at the lowest temperature possible consistent with their ultimate use (table stock or chipping). Most fungi do not grow much at temperatures of 38 F or lower, but some development will occur at higher temperatures.

Chemical control in storage

Several commercial storage products, like Phostrol (sodium, potassium and ammonium phosphates) at 0.1 gallons per ton and Storox (hydrogen peroxide/peroaxyacetic acid mixture) at 5 fluid ounces per 100 gallons of water, are registered for control of storage pathogens. Note that 0.5 gallons per ton is the recommended product plus application rate for tubers. Stadium, a new product from Syngenta Crop Protection, is now registered for use as a pre-storage treatment for management of Fusarium dry rot and Silver Scurf in storage. Stadium is a three-way mixture of azoxystrobin (Quadris), fludioxonil (Maxim) and difenoconazole (Inspire) and the application rate is 1.0 fluid ounces per 20 cwt (ton) of potato tubers carried in 0.5 gallons of water per ton of tubers.

Results from trials at MSU and University of Idaho show that Stadium provided effective disease control over several years of testing. Results from testing at MSU indicated the product also suppressed tuber late blight, although this is not on the label. It is likely that conditions have also been conducive for the perennial and ubiquitous Fusarium dry rot pathogens, of which 13 species are known in Michigan.

Conclusion

In conclusion, although P. infestans is not known to overwinter directly in soil in spore form, it does survive in infected potato tubers. Over the past 15 years of monitoring, volunteer tubers have survived in most parts of Michigan. The likelihood of survival will be updated for 2016 on April 1 the Michigan Late Blight Risk Monitoring website. Growers should be prepared to protect against late blight again next season.

Dr. Kirk’s work is funded in part by MSU’s AgBioResearch.