Potatoes in Canada

Features Agronomy Diseases
Late-season management of late blight


Aug. 29, 2014 – Potato late blight is a devastating disease of potatoes. It can destroy a potato field in a few days if wet weather prevails and no effective fungicides are applied. This is a community disease because late blight spores are spread by wind from infected to healthy fields. Thus, the management practices followed by individual growers will affect an entire potato production area.

Late blight is carried over from one season to the next by infected tubers: seed potatoes, cull potatoes or volunteers. The best defense is to avoid or delay the establishment of late blight. Planting healthy seed, destroying cull piles and volunteers, starting to spray early and scouting fields regularly should allow you to manage late blight without serious losses. 

Scout fields regularly 
Even small amounts of foliar blight may lead to significant tuber infection if conditions are favorable during harvest. Although late blight is more likely to spread from field to field by wind rather than on scouts, it is unwise to take risks. Always wear boots that can be disinfected between fields with products such as diluted household bleach (mix one part bleach to nine parts water). Disposable boots and pants are also available. Scout healthy fields first and infected fields last. 

Scout risky areas twice a week. 
Risky areas are low spots, compacted areas, rows close to tree lines, field edges along creeks or ponds, pivot center points, pivot wheel tracks, weedy areas and any area in the field with poor air movement that allows the foliage to remain wet for extended periods after rain or irrigation. Scout these areas thoroughly. 

If late blight is found, destroy hot spots 
The late blight fungus is capable of producing as many as 100,000 spores from a spot on a leaf the size of a quarter. If the disease is spotted early, disking or spraying small patches with a fast-acting desiccant can remove a source of spores and slow the spread of the disease. Leaf symptoms show up three to five days after infection, so kill an area at least 50 feet beyond the infected patches. Destroying patches does not always stop or slow the spread of late blight, particularly if the weather has been cool and wet, but it is usually worth trying if there are only a few hot spots in a field. Map infected areas of fields and check them regularly for tuber infection.

Kill vines completely, at least two weeks prior to the anticipated harvest date. 
The late blight fungus needs living tissue – stems, green foliage or tuber tissue – to survive and produce spores. Infected vines mixed with tubers during harvest under cool, wet conditions may lead to tuber infections that are not visible until later in storage. 

Waiting at least two weeks after top kill minimizes the risk of tuber infection during harvest and allows infected tubers to decompose in the field. 

Cultural control during harvest and storage
Avoid harvesting during wet conditions. Tubers can become infected at harvest even with minimal foliar blight if conditions are wet. Harvesting during rain is particularly dangerous. Harvesting when skins are mature reduces the likelihood of tuber infection as does minimizing skinning, cuts, and shatter bruises. Any break in the tuber skin provides an ideal place for late blight and other diseases to enter tubers. Although the late blight pathogen does not need a wound to infect tubers, any damage to the skin makes infection more likely. Wet tubers are also more likely to be infected. 

Assess the level of tuber infection and grade out suspicious tubers 
Sort tubers during harvest and remove as many decayed tubers as possible. Identifying tubers infected with late blight can be difficult, especially if tubers are covered with soil. Save suspect tubers and get them checked by someone experienced in identifying late blight. 

A crop with more than three per cent late blight infection may be very difficult to store. Even very low infection levels can lead to serious losses in storage if storage conditions are not ideal, so it is important to determine the level of infection. 

1. If infected areas are harvested. Grade out all suspicious tubers and put the healthy-looking crop near the storage door. Remove and market the tubers immediately if any tuber decay becomes evident. 

2. Consider not harvesting infected areas. At the end of the season, dig the areas that are not harvested and lay the tubers on top of the ground. Freezing during the winter will kill infected tubers. 

Apply a post-harvest treatment when storing potatoes 
Phosphonates (phosphorous acid or phosphite-based products) help keep healthy tubers from being infected in storage. Good tuber coverage is essential. Phosphonates need to be sprayed evenly over the tubers when they are rolling under the spray bar. 

Phostrol, Rampart and Confine are labeled for post-harvest treatments. Confine is not recommended for use on potatoes intended for seed, as sufficient data does not exist to support this use.

Carefully monitor and regulate storage conditions 
It is critical to pass high volumes of air flow throughout the pile to cool and dry the tubers during the early storage period. Remove vines, loose soil, and anything else that may interfere with air distribution in the pile when loading the bins. 

If foliar late blight was present in the field prior to harvest, it is important to ventilate the storage as quickly as possible with a high volume of air. Begin supplying air to the pile to dry wet potatoes and to equilibrate the pile temperature as soon as the first two to three air ducts are covered. 

It may be necessary to run fans continuously with reduced or no humidity until tubers are dry. Expect increased pressure bruising and shrinkage losses to potatoes subjected to these storage conditions, especially if the tubers are not marketed early 

Check piles for wet spots (tuber rot) as soon as the potatoes are put in storage. If wet spots develop, supply additional air to those areas and plan on removing the potatoes as soon as possible. Holding potatoes at lower temperatures (below 45 F) may lessen development of disease in storage, but this can have a significant impact on the market use of the potatoes. 

Consider the end use of the tubers prior to manipulating storage temperatures. 

Spraying the crops
Ensure thorough and complete fungicide coverage in the potato canopy. A preventative fungicide program needs to start early to ensure that new growth is protected when infection periods occur. 

The crop canopy must be completely covered with no skips or untreated areas. Fungicides must be applied at the appropriate interval for the disease pressure in your area. 

The water volume and pressure used should provide good leaf coverage and penetration of fungicide into the canopy. 

Recalibrate the sprayer often and replace nozzles that are under or over applying by more than 10 per cent of the boom average. Raise the boom height as the crop grows to maintain proper overlap of the spray pattern. Pressure wash farm equipment after use in an infected field.

There are several late-blight-specific and broad-spectrum fungicides that are effective in reducing late blight infection. Consult OMAFRA publication 838 for a complete list of these fungicides. Read the label for specific application information 

Based on field experience, an effective tank mix to spray when late blight is found is Curzate + Bravide + Kocide or Parasol. This tank mix should be applied immediately after late blight is detected followed by a second application three to four days after the first one. 

The copper is washed off the foliage and into the soil after a rain. It remains in the soil acting like a barrier protecting tubers against germinating late-blight spores. 

After two applications of the tank mix, keep spraying the crop at intervals no longer than five days. 

The list of registered fungicides specific for late blight includes several good protectants such as Revus, Tattoo-C, Ranman etc. Late-blight-specific fungicides should be tank mixed with an EBDC (e.g. Dithane, Polyram) to reduce the risk of resistance and to control early blight. 

Phostrol as a foliar spray is labeled for preventative control of late blight and preventative suppression of pink rot. The label indicates that mixing this product with certain surfactants or foliar fertilizers can cause crop injury. Determine crop sensitivity by applying the combination to a small area of the crop and evaluate three to seven days later for adverse effects. 

Regardless of the fungicides used, spray timing and spray coverage are essential to reduce late blight incidence. 


August 29, 2014
By Eugenia Banks