MSU Extension

Several more fields in the southern counties of Michigan have reported late blight over the past week. The amount of disease is generally light, although in some fields lesions can be found in several areas of the field. Areas in fields that are particularly vulnerable are field margins, especially those close to tree lines and where water can accumulate such as around pivot tracks and tractor wheel lines.

The genotype of the Phytophthora infestans isolate responsible for all late blight confirmations in potatoes and tomatoes has been the US-23 genotype. This was confirmed by GPI allozyme analysis. This genotype is Ridomil-sensitive; however, the recommendations for treatment include treating with one of the translaminar fungicides.

Conditions remain conducive for late blight in irrigated potato crops despite the high temperatures experienced over the last few days. Forecasts and DSV accumulations can be checked daily at the Late Blight Risk Monitoring website. For example, most parts of Michigan have high risk forecast for the next five days.

Michigan State University Extension advises growers to continue scouting on a weekly basis where late blight has not been reported and treat fields aggressively in areas where it has been reported (see Table 1 for recommendations). Growers should sample symptomatic plants and send them for diagnosis and continued application of residual protectant fungicides. Recommendations for appropriate late blight control where late blight has been confirmed or is suspected should include approaches suggested in Table 1 and include desiccation of infected areas.

Table 1. Suggestions for appropriate fungicides for late blight control including semi-systemic fungicides under different late blight conditions in susceptible potato varieties

fb = followed by
GLA = Green Leaf Area
* TPTH has 7-day post-harvest interval (max 11.25 oz/Acre/season).
** Chlorothalonil has 7-day post-harvest interval.
*** Protectant applications of an EBDC or chlorothalonil-based fungicide should be maintained on a 5-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.

The indication of high risk was communicated early in the season to the industry. This included information from MSU that winter temperatures were likely to have resulted in a high likelihood that volunteer tubers would have survived the winter; 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 in my opinion resulted in many acres of potatoes being spared from late blight in 2013.

The early action of the Michigan Potato Industry Commission in conjunction with MSU to contact growers also alerted growers to the possibility of late blight risk in 2013, which would have had a mitigating effect on the progress of the disease. One further fact for 2013: for all areas of Michigan, the accumulated late blight disease severity values for 2013 are among the highest we have recorded in the past 10 years.

Late season disease biology and recommendations

Sporulation in this pathogen is favored by wet weather with moderate temperatures (60 to 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.

Symptoms

The first symptoms of late blight in the field are small, light to dark green, circular to irregularly-shaped, water-soaked lesions (see the Michigan Potato Diseases: Late Blight bulletin for pictures). 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. Leaf veins do not limit lesions, 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 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.

Late season disease cycle

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 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. 

Sporangia may germinate at temperatures between 44 to 55 F when free water is present on leaves and form eight to 12 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. 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 more likely to be infected.

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.

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. Ridomil Gold Bravo or other Ridomil combination products may also be used for this strain of late blight. 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. Other new, very effective products include Revus (Group 40, mandipropamid) and Ranman (Group 21, cyazofamid).

Applied within a protectant program, all of these products give excellent late blight 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) or 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 the 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 and not be used late in the season.

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 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.

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