MSU Extension

Potato late blight has been confirmed in commercial potatoes in Wisconsin in northern Adams County. University of Wisconsin plant pathologist Amanda Gevens confirmed that “yesterday, June 23, 2015, late blight was confirmed in a commercial field in northern Adams County. The field was only at about 40 percent emergence and the disease appeared to have been initiated by seedborne inoculum.”

No late blight has been reported to date in Michigan, although there are confirmed reports from North Carolina and Florida in the eastern sector of the United States. Conditions remain conducive for late blight development and most areas are now past the 18 disease severity value (DSV) thresholds for late blight and the 300 P-day thresholds for early blight. See the station comparison page on the Michigan Late Blight Risk Monitoring website.

Potato late blight, caused by the water mold Phytophthora infestans, has the potential to be a very destructive disease of Michigan potatoes. The pathogen favors 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 soft rot of tubers often occurs in storage following tuber infections.

Symptoms

The first symptoms of late blight in the field are small, light to dark green, circular to irregularly shaped water-soaked lesions. A full description with pictures can be found in Michigan State University Extension Bulletin E2945, “Michigan Potato Disease: Late Blight.” Lesions usually first appear on the lower leaves where the microclimate is more humid. However, they may occur on upper leaves if weather conditions are favorable and the pathogen has been carried into the field by air currents. Lesions often begin to develop on the compound leaf near the point of attachment to the petiole, which is often cupped, or edges, where dew is retained longest. During cool, moist weather, lesions expand rapidly into large, dark brown or black spots, often appearing greasy. As new infections occur, and existing lesions coalesce, entire leaves may become blighted and killed within a few days. On stems, lesions are often initiated at the point of attachment to the stem and leaves become detached shortly after infection. The lesions continue to develop along the length of the stem and even in hot dry weather can remain active.

In the early morning or during cool damp weather, a white, velvety growth may be seen on the underside of infected leaves. This white velvety growth distinguishes late blight from several other foliar diseases of potato. A pale green to yellow border is also often present around lesions. Plants severely affected by late blight also have a distinctive odor resulting from the rapid breakdown of potato tissue. This odor is similar to that produced by chemical vine-kill or after severe frost.

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. 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.

Positive identification of late blight can be made by microscopic examination of lesions from infected leaves or tubers collected when the fungus is producing spores. The water mold can be quickly identified by the distinctive size and shape of the spores and spore bearing stalks.

Disease cycle

Phytophthora infestans, the causal agent of late blight, is not a true fungus but a water mold belonging to the phylum Oomycetes. Oomycetes like P. infestans form large, clear lemon-shaped spores called sporangia on stalks called sporangiophores. Though they are relatively large in comparison to those of true fungi, they cannot be seen without the aid of a microscope that can magnify at least 100 times. The sporangiophores have distinct periodic swellings at points where sporangia were produced.

Sporangia may germinate at temperatures between 44 and 55 F when free water is present on leaves and form eight to 12 motile zoospores per sporangium. These swim freely in water films, attach to the leaf surface (encyst) and infect the plant. Encysted zoospores infect leaves by penetrating the leaf surface with a germ tube, either through stomata (breathing pores) or by means of direct penetration. 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 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 the winter (volunteer potatoes). In spring, the pathogen can be transmitted from infected tubers in cull piles or volunteers to potato foliage by airborne spores. Infected seed potatoes are also an important source of disease. Some infected tubers may rot in the soil before emergence, and not every potato that emerges from an infected tuber will contract late blight. Sporangia of P. infestans 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.    

Monitoring and control

Effective management of this disease requires implementation of an integrated disease management approach. Although the most important measures are cultural, resistant cultivars and chemical controls should also be utilized.

Monitoring and disease forecasting

Efforts must be made to closely monitor crops for the incidence of disease. Scouting for signs of late blight should begin as soon as green tissue emerges. When a canopy develops, look for late blight in the lower portions of the plant where the foliage stays wet longer. Scouting efforts should also be concentrated in areas of the field most likely to have high moisture, dew or relative humidities for the greatest length of time or areas missed by fungicide applicators. Low spots where soil moisture is highest and parts of the field shaded by windbreaks are examples of areas where scouting should be intensified. Care should be taken not to spread late blight from field to field when scouting. Rubber boots, which can be washed after leaving a field, should be used if disease is present. Late blight inoculum can also be spread from field to field on equipment, so this should also be washed after leaving the field. As the risk of disease becomes greater, crop monitoring should be intensified. As soon as the disease is detected, other chemical interventive control measures should be used.

Computer-based programs, like USAblight, are available to track weather conditions and help predict or forecast when the disease may occur. Disease forecasting programs predict when environmental conditions are most favorable for disease and recommend when fungicide applications should be applied. Disease forecasting systems express the effects of temperature and relative humidity on disease development as “severity values.” Local late blight forecasts are available for many potato production areas in Michigan from the Michigan Late Blight Risk Monitoring website. Unless you know the forecast information is appropriate for your fields, forecasts should only be used as a general indication of how favorable weather has been for late blight.

Resistant cultivars

No potato cultivars are immune to late blight and most cultivars planted in Michigan are susceptible. However, several moderately susceptible cultivars (e.g. Pike, and some FritoLay varieties) are available and could be planted if late blight is expected to be a problem.

Cultural control

Cultural practices are the first line of defense against late blight. Before planting, growers should take several measures to control late blight. First, it is important to eliminate sources of inoculum. The initial sources of inoculum are likely to be infected potatoes in cull piles, infected volunteer potato plants that have survived the winter, and infected seed tubers. Therefore, it is important to keep a clean operation by destroying all cull and volunteer potatoes. Cull piles should be kept as small as possible as piles of about 500 cwt do not freeze throughout the pile. Piles should be covered with plastic tarpaulins to increase the temperature within the piles in the fall and accelerate breakdown. Waste potatoes can be spread onto fields not for potato production in the fall at a rate of about 400 cwt per acre as supplemental fertilizer and after spreading the tubers should be pulverized and left near or on the surface of the field to allow them to freeze. Rock piles that are deposited after planting contain rocks and potato seed pieces and should also be monitored carefully throughout the growing season. Emerging plants should be killed off with Roundup.

Although too late for 2015, for subsequent years it is also very important to make sure you only plant certified seed. Use of seed saved from local crops may increase the risk of late blight. Seed sources should be selected carefully to avoid bringing in late blight on seed, especially new strains of the pathogen. Look for characteristic brown discoloration of the potato flesh under the skin of seed tubers. Any tubers suspected of being infected with late blight should be tested to confirm its presence. Contaminated loads of seed should be rejected. Second, avoid conditions that favor late blight. Weather conditions strongly influence the incidence and severity of late blight. Although weather conditions are beyond control, field selection and carefully managed irrigation practices can help reduce the extent of periods favorable for disease development. Fields with good water infiltration and drainage characteristics are desirable for planting potatoes. If irrigation is applied, try to apply water between midnight to 8 a.m. to avoid prolonging the length of time leaves are wet. Leaves are often wet with dew during these hours anyway, so irrigation during this period does not unduly prolong the leaf wetness period. Alternatively, apply irrigation during daylight hours, beginning after leaves have been dry for at least two hours and ending two hours before dark, again so leaves have a dry period before and after irrigation. Most disease-causing spores are released into the air between 9 a.m. and 1 p.m.

After planting and early in the season, it is important to get rid of cull potatoes and potato pieces resulting from seed cutting operations or left after loading or unloading at storage facilities as these may support the production of inoculum whether or not the pieces are sprouting. It is also beneficial to control weeds and alternative late blight hosts such as hairy nightshade that may contribute to disease spread under some conditions. Although weed species are not late blight hosts, they can contribute to conditions that favor disease development by restricting air movement within the canopy. Heavy weed infestations also prevent adequate coverage of potato foliage with fungicides.

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.

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 and oomycetes do not grow much at temperatures of 38 F or lower, but some development will occur at higher temperatures.

Chemical control

Late blight protection programs should be based on a residual protectant fungicide such as chlorothalonil or mancozeb. However, as there may be a shortage of chlorothalonil products in 2015, a base alternative product might be necessary. Under high disease pressure situations, the programs incorporating Revus products, Forum, Curzate 60DF, Ranman, Tanos, Gavel, Ariston, Zing! or Previcur Flex should be used. Ridomil-based products have proved very effective in recent years where the genotype of P. infestans has changed to one that is metalaxyl-sensitive as with US-23 which has predominated in Michigan since 2013. In Michigan, Headline and Quadris have given very useful late blight control, but these products should be used in strict adherence with anti-resistance development strategies, i.e., always mix with a protectant fungicide. Consult your local advisor for appropriate rates and additional combinations. These products must be used in combination with protectant materials such as EBDC or chlorothalonil-based products.

Destruction of areas within crops with late blight should follow the rules that 30 rows either side of the newest lesions at the border of the late blight locus and 100 feet along the row (either side) are killed with Reglone or with Gramoxone. Although harsh, MSU trials have shown the latent period between infection and symptom development is about seven days and although not visible, plants within this area are already infected.

In seasons when the severity of weather conditions would not favor severe late blight development programs based on chlorothalonil (e.g., Bravo WS 6SC, Echo 6SC, Equus 6SC or other formulations), EBDC (e.g., Dithane 75DF, Manzate 75DF, Manex 4FL, Penncozeb 75DF, Polyram 80WP) will reduce the risk of the establishment of the disease. The addition of TPTH 80WP to any of the protectant programs would enhance disease control particularly towards the end of the growing season. (TPTH 80WP has a seven-day pre-harvest interval, also note maximum use rate since 2002 is 11.25 ounces per season.) Fixed copper-based products, such as Champ and Kocide, can also be used in protectant programs. These products are best used early in programs or immediate post-harvest for killing spores perhaps from adjacent crops and should always be applied at the full recommended rate of application. The observations of individuals responsible for implementing programs should determine when best to change from one product to another.

Of major note is that the Fungicide Resistance Action Committee (FRAC) have specific recommendations for mixing fungicides with high risk of resistance development. Fungicides are now labeled with a Group number, e.g., Headline, Tanos, Quadris, Gem are all Group 11; these fungicides should be not mixed or immediately alternated in a fungicide-based protectant program. The application of these fungicides as stand-alone products has never been recommended by MSU for late blight control. They should always be mixed with a protectant surface residual fungicide.

The appropriate placement of translaminar and other systemic products within programs is determined by the mode of action of the product in relation to host and disease development, but all products are best used within a preventative protectant program. For example, Previcur, Acrobat, Quadris, Headline, Gem, Gavel or Curzate may be applied to protect new growth early in development. Curzate and Previcur Flex may be applied while the canopy is expanding, but before senescence and Forum is most effective during canopy expansion and as a post-senescence product and can be applied up to late crop senescence.

Recommended programs for late blight control are not straightforward. The product of choice may well depend on how and from where the disease has developed. Some possible scenarios are shown in the table below where a range of containment procedures is described for susceptible varieties and different levels of disease in the field.

fb followed by;
GLA Green Leaf Area
* TPTH has seven-day post-harvest interval (max 11.25 ounces per acre per season).
** Chlorothalonil has seven=day post-harvest interval.
*** Protectant applications of an EBDC or chlorothalonil-based fungicide should be maintained on a five-day schedule until the vines are completely dead.
**** Infected areas should be treated last and a fungicide should be applied during the exit from the field.

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