Late blight found in potatoes in Michigan’s Montcalm County

Late blight was found in Michigan potatoes Aug. 22, 2017. Conditions remain conducive for late blight in irrigated potato crops.

Photo 1. Lesions developing near the petiole.

Photo 1. Lesions developing near the petiole.

Late blight was found in potato crops near Edmore, McBride and Vestaburg, Michigan, in Montcalm County Aug. 22, 2017 by ABC Consultants scouts. Initial genotyping confirmed isolates as US-23 by GPI isomerase testing, conducted by the Michigan State University Potato and Sugar Beet Pathology Laboratory. Mating type and Ridomil sensitivity are underway.

Growers should be aggressively treating all fields on a minimum five-day schedule in Montcalm County. Applications should include a translaminar fungicide in combination with a protectant (chlorothalonil plus mancozeb) and Super Tin. Although a few fungicides have yet to be added, you can view the current fungicide rates.

Areas in fields that are particularly vulnerable are field margins, especially those close to tree lines, raised cable lines and where water can accumulate such as around pivot tracks and tractor wheel lines. Recommendations for late blight treatment remain the same as in previous reports posted at MSU Extension and include treating with one of the translaminar fungicides listed at the Michigan Late Blight Risk Monitoring website.

Conditions remain conducive for late blight development. See the station comparison page at Todays Lateblight Forecast. Forecasts and disease severity value (DSV) accumulations can be checked daily at MSU’s Late blight Forecast page. Continued monitoring for late blight is recommended.

National late blight updates can be found at USA Blight. This resource provides updated information on the detection and characterization of late blight on potato and tomato crops in the U.S. This year to date, late blight has been confirmed on tomato or potato in the states of Connecticut, Florida, Massachusetts, Maine, New York, North Carolina, Pennsylvania, Virginia, Washington and Wisconsin, and Manitoba and Ontario, Canada.

For the reported cases that have been genotyped, the pathogen was either US-23 or US-8 (Washington and Wisconsin). The US-23 clonal lineage has been reported in Michigan and most of the U.S. in recent years and in general is still managed well with phenylamide fungicides. There has also been reports of the US-8 clonal lineage in parts of the U.S. in recent growing seasons. The US-8 is resistant to the phenylamide class of fungicides.

Continue scouting on a weekly basis where late blight has not been reported and treat fields aggressively in areas where late has been reported (see Table 1 for recommendations). 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.

The indication of high risk was communicated early in the season to the industry; this included information from the university that winter temperatures may have resulted in a high likelihood that volunteer tubers would have survived the winter; feedback from scouts that survival of volunteer potatoes was profuse.

Disease biology and mid-season recommendations

Sporulation in this pathogen is favored by wet weather with moderate temperatures (60 – 80 degrees Fahrenheit), high humidity and frequent rainfall. Under these conditions, the disease can spread 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 MSU Extension bulletin E2945, “Michigan Potato Diseases: Late Blight”) 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 via 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 (Photo 1).

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 (Photo 2). 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.

late blight in montcalm

Photo 2. Lesions that are not limited by leaf veins.

In the early morning or during cool, damp weather, a white, velvety growth may be seen on the underside of infected leaves (Photo 3). 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 (Photo 4). 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 in montcalm

Photo 3. Sporulating late blight lesion with white, velvety mycelial growth.

late blight in montcalm

Photo 4. Yellowing, light green and necrotic lesions developing on the leaf surface.

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 typically survives from one growing season to the next in infected tubers in storage, potato cull piles or infected tubers missed during harvest that remain viable 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 potato seed is also an important source of disease. Some infected tubers may rot in the soil before emergence, and not every potato that emerges from infected seed will develop 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 from 44 to 55 F when free water is present on leaves, and eight to 12 zoospores form 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 temperatures of 55 to 70 F, sporangia germinate by means of a single germ tube.

Overnight 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. Thus, tubers near the soil surface are more likely to be infected.

Recommendations

In mid- and late season when rain is frequent, 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 vines will remain green 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 also may harbor inoculum that can infect tubers at 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 proximity of the field at harvest, it may also be beneficial to apply labeled fungicides to foliage after vine killing to kill viable 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 maintained so 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 38 F or lower, but some development will occur at higher temperatures.

Chemical control

Under conducive environmental conditions and high disease pressure, fungicide programs incorporating Revus products, Forum, Curzate 60DF, Ranman, Tanos, Gavel, Zampro or Previcur Flex should be used. Ridomil Gold Bravo SC or other Ridomil combination products may also be used for the US-23 strain of late blight; however, experimental research field trials at MSU have previously shown that curative applications of any fungicide tend to fail. 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.

Gavel (zoxamide plus mancozeb, Gowan) is also best used as a protectant and has been reported to reduce tuber blight. Destruction of areas within crops infected with the late blight pathogen 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 Gramoxone. Although these are extreme measures, trials at MSU have shown the latent period between infection and symptom development is about seven days, and although disease symptoms may not be visible, plants within this area are already infected.

In seasons when weather conditions do 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 disease establishment. 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 immediately 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—for example, Headline, Tanos, Quadris and Gem are all Group 11. These fungicides should be not mixed or immediately alternated in a fungicide-based protectant program. The application of all these translaminar 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 spray programs is dictated by the product mode of action 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 applied as a post-senescence product and up to late crop senescence.

Recommended programs for late blight control are not straightforward. The product of choice may be dependent 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.

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

Disease category

Late maturing especially storage varieties

No senescence – early senescence

Mid – late senescence

A) None

Curzate or Tanos or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil

Quadris or Headline or Gem + EBDC or Gavel

Omega

Champ or Kocide can be added to enhance activity

Curzate or Tanos or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil+

Supertin/Agritin+ EBDC or chlorothalonil

Chlorothalonil or Gavel (various + ZN)

Omega

Champ or Kocide can be added to enhance activity

B) Few random lesions, even distribution throughout field (0 – 1 percent foliar infection)

Curzate or Tanos or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil+TPTH

Or Chlorothalonil followed by EBDC+TPTH or Chlorothalonil + TPTH 5day

followed by Chlorothalonil (various + ZN) 

Or Gavel

If metalaxyl sensitive strain found then use a Ridomil Gold-based product

Curzate or Tanos or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil+TPTH

Or Chlorothalonil followed by EBDC+TPTH or Chlorothalonil + TPTH 5 day

followed by Chlorothalonil various + ZN) 

Or Gavel

If metalaxyl sensitive strain found then use a Ridomil Gold-based product

C) One or more (up to 5) loci spreading from the edge of the field or from several centers within the field (1 percent overall field infection, but locally heavily infected plants 5 – 10 percent)

Curzate or Tanos or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil+TPTH

Kill infected area with Reglone****

followed by EBDC+TPTH or Chlorothalonil + TPTH every 5 days until vines dead

If metalaxyl sensitive strain found then use a Ridomil Gold-based product

Curzate or Tanos or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil+TPTH

Kill infected area with Reglone

followed by EBDC+TPTH or Chlorothalonil + TPTH every 5 days until vines dead

If metalaxyl sensitive strain found then use a Ridomil Gold-based product

D) Partial crop infection large areas infected with up to 20 percent loss of green leaf area evenly distributed throughout the field or large areas of the field

Curzate or Tanos or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil+TPTH

Chlorothalonil (various + ZN) + TPTH kill infected area with Reglone

followed by EBDC+TPTH or Chlorothalonil + TPTH every 5 days until vines dead

If metalaxyl sensitive strain found then use a Ridomil Gold-based product

Curzate or Tanos or Forum or Previcur Flex or Revus or Ranman + EBDC or chlorothalonil+TPTH

Chlorothalonil (various + ZN) +TPTH kill infected area with Reglone

followed by EBDC+TPTH or Chlorothalonil + TPTH every 5 days until vines dead

If metalaxyl sensitive strain found then use a Ridomil Gold-based product

E) 20-100 percent crop infection with large loss of green leaf area***

Kill infected area with Reglone

followed by EBDC+TPTH or Chlorothalonil + TPTH every 5 days until vines dead

Kill infected area with Reglone

followed by EBDC+TPTH or Chlorothalonil + TPTH every 5 days until vines dead

* 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. Rosenzweig’s work is funded in part by MSU’s AgBioResearch.

Related Articles