by Noah Rosenzweig and Chris Long, Michigan State University, Department of Plant, Soil and Microbial Sciences; Sandesh Dangi and Phill Wharton, University of Idaho
Late blight was found in a potato plant in St Joseph County on July 7, 2017. Initial genotyping confirmed isolates as US-23 by GPI isomerase testing, but has not been confirmed by the Michigan State University Potato and Sugar Beet Pathology Laboratory.
Recommendations for late blight treatment include treating with one of the translaminar fungicides listed at the Michigan Late Blight Risk Monitoring website.
The indication of high risk was communicated early in the season to the industry due to potential volunteer survival. This included information from the university that winter temperatures were likely to have resulted in a high likelihood that volunteer tubers would have survived the winter. Conditions remain conducive for late blight development and some areas are now past the 18-disease severity value (DSV) thresholds for late blight.
See the station comparison page on Today’s Lateblight Forecast at — forecasts and DSV accumulations can be checked daily. Continued monitoring for late blight is recommended. Sample symptomatic and suspicious plants and send them for diagnosis to the MSU Potato and Sugar Beet Pathology Laboratory for proper and accurate genotyping and fungicide sensitivity testing. This service is provided for free. Identification of genotyping is important to identify effective fungicide chemistries. Fields with late blight and adjacent fields should be on a five-day application schedule.
National late blight updates can be found at USAblight. This resource provides updated information on the detection and characterization of late blight on potato and tomato crops in the U.S.
As of July 10, 2017, late blight has been confirmed on tomato or potato in Florida, North Carolina, Virginia, Washington and Ontario, Canada. For all the reported cases that have been genotyped, the pathogen was US-23. This has been the predominant clonal lineage in Michigan and most of the U.S., in recent years and in general is still managed well with phenylamide fungicides.
Late blight of potato, caused by the water mold Phytophthora infestans, has the potential to be a very destructive disease of potato in Michigan. The pathogen favors wet weather with moderate temperatures (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.
The first symptoms of late blight in the field are small, light to dark green circular to irregularly-shaped water soaked lesions. These 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. Lesions are not limited by leaf veins and, if formed at leaf tips or edges, they can cause young expanding leaves to be misshapen. 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 between 44 and 55 F when free water is present on leaves and form eight to 12 motile zoospores per sporangium. These zoospores 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 55 to 70 F, sporangia germinate by means of a single germ tube. Night 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 survives from year to year in infected tubers placed in storage, in piles of cull potatoes or infected tubers missed during harvest that remain unfrozen over the winter (volunteer potatoes). In the 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 Phytophthora 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 humidity for the greatest length of time or areas missed by fungicide applicators. Low spots where soil moisture is highest and area 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 control intervention measures should be used.
Computer-based programs (e.g., Blightcast) 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 Today’s Lateblight Forecast. However, accurate weather data are required for accurate disease prediction, and this is best achieved with weather monitoring machines in each field, especially if fields vary greatly in a specific location and topography.
Unless you know the forecast information is appropriate for your fields, the 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. The MSU-bred cultivar Jacqueline Lee (table stock) is highly resistant to the US-8 genotype of late blight and is currently being used as a parent for the production of new chip and processing cultivars. 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, take several measures to control late blight. 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, 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 hundredweight 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 in the fall at a rate of about 400 hundredweight 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. Kill off emerging plants with Roundup. Make sure you only plant certified seed. Use of seed saved from local crops may increase the risk of late blight.
Carefully select seed sources 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.
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 during the hours of 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 the hours of 9 a.m. and 1 p.m.
After planting and early in the season, 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 regardless of if the pieces are sprouting. Control weeds and alternative late blight hosts such as hairy nightshade, which 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, 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 near 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 do not grow much at temperatures of 38 F or lower, but some development will occur at higher temperatures.
Chemical control
Under high disease pressure situations, the programs incorporating Revus products, Forum, Curzate 60DF, Ranman, Tanos, Gavel or Previcur Flex should be used. 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 and never apply consecutive treatments. 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. New products of note include Tanos [Group 11, duPont, 25 percent cymoxanil (as in Curzate) + 25 percent famoxadone)], which should be applied at 6.0 ounces per acre (no more than six applications per year and mixture with Manzate or chlorothalonil recommended; do not mix or follow with a Group 11 fungicide, e.g., Quadris, 14-day pre-harvest interval).
For early blight and white mold, use Endura (Group 7, BASF, 70 percent Boscalid, no more than four applications per season maximum of 20.5 ounces per acre per season, 30-day pre-harvest interval). Other effective products include Revus (Group 40, mandipropamid) and Ranman (Group 21, cyazofamid).
Applied in conjunction with a protectant program, these products give excellent late blight control. In addition, trials over several years at MSU have shown Quadris and Headline are still moderately effective for early blight control and Omega for white mold control. Gavel (zoxamide + mancozeb, Gowan) is also best used as a protectant and has been reported to reduce tuber blight.
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, trials at MSU have shown that 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.
The Fungicide Resistance Action Committee (FRAC) has 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 Table 1 where a range of containment procedures is described for susceptible varieties and different levels of disease in the field.
This article was published by Michigan State University Extension. For more information, visit