2010 Late Blight Strategy

Introduction

Late blight, caused by the fungus Phytophthora infestans, is one of the most devastating diseases of potatoes. If weather conditions are favorable and no effective fungicides are applied, late blight can destroy a potato field in 4 or 5 days. To prevent this, a management strategy is necessary based on preventing or delaying initial infection and slowing the rate of disease spread once late blight is established.

Fungicides must be used effectively to control late blight. Spray applications should be based on weather conditions, crop growth and disease pressure. Short intervals of 3 to 5 days are necessary when late blight has been found and the weather is cool and damp.
Intervals can be extended when weather is warm and dry. Fungicides with different modes of action need to be rotated to avoid the development of resistance. Good coverage is essential for fungicides to be effective.

Late blight is a community disease. Once the disease starts, it may be spread long distances by wind affecting many farms and many fields. A coordinated approach to disease control is best.

The Spread of Late Blight

Late blight is carried over from one season to the next in living tissue. Healthy seed is important because it reduces early sources of spores. Late blight may also be carried over in cull piles and volunteers, so these sources of disease need to be eliminated.

Tomatoes and potatoes growing in home gardens may be sources of late blight.

Some common weeds related to potatoes - hairy nightshade and bittersweet - are also susceptible to late blight. These weeds growing in hedgerows may serve as a source of spores to spread late blight.

Spores are carried long distances by wind. Rain splash and farm equipment can spread spores within fields.

Weather Conditions and Disease Development

Late blight is favored by cloudy, wet weather and cool temperatures. Spores need a film of water on potato foliage to germinate. At temperatures of 13 C-21 C each spore germinates directly, penetrates the plant and causes one lesion. At cooler temperatures between 7 C-12 C and in the presence of free water, each spore produces 8 zoospores that swim in the water film. Each zoospore can initiate infections.

Disease symptoms develop 3 to 5 days after the initial infection. Once lesions form, a single foliar lesion can produce as many as 700,000 spores that spread late blight.

Late Blight Management Practice

• Symptoms on leaves - The first symptoms on leaves are small, light to dark green water-soaked spots. During cool, wet weather, lesions expand rapidly into large dark brown or black spots usually surrounded by a light green halo. A white fungal growth usually develops on the underside of affected leaves on the light green halo.
  
• Symptoms on stems - Dark brown irregular lesions form on stems. These lesions usually initiate at the point of leaf attachment. The white fungal growth may develop on stems under favourable weather conditions.
• Symptoms on tubers - Tuber infection is characterized by irregularly shaped, brown to purplish areas on the skin. These infected areas appear slightly depressed. A dry, granular reddish- brown develops under the skin.Destroy Cull Piles Prior to Crop Emergence

Tubers near the top of a cull pile will normally break down and rot, but those in the middle of the pile may sprout and grow in the spring. Eliminate cull piles by burial, freezing or feeding to livestock.

• Burial - When burying culls, make sure the pile is not buried near a water source. Check that the water table is low, and cover the tubers with at least 1.8 metres (six feet) of soil to prevent re-growth. Moving potatoes to a burial site can be expensive. Keep the distance traveled as short as possible. Check the cull site during the summer for volunteers. Pull or spray any plants that appear with herbicide. Add extra soil if the cull pile settles during decomposition.
• Freezing - Spread a layer of cull potatoes one tuber thick on fields in late fall or winter to ensure that the potatoes freeze. Use fields that will not be used to grow potatoes in the future to avoid the spread or build-up of soil-borne diseases like common scab.
• Covering cull piles - This method can be used on cull piles late in the spring when hard frosts are unlikely or when immediate action needs to be taken. Spread the culls out evenly, and cover them with plastic, preferably black polyethylene. The tubers will heat up and break down. Make sure the entire pile is covered so there is no chance of plant growth or spore escape. One of the problems with this method is the watery mess that will form in and near the pile. Sawdust or other absorbent material can be used to soak up the potato runoff and then removed from the site. Covered piles should not be placed near waterways.
• Use as livestock feed - Raw potatoes can be fed to cattle. It is estimated that 4 pounds of potatoes equals one pound of barley in nutritional content.

Field Selection

• Select well drained fields - In fields with good water infiltration, water does not accumulate between rows after a rain. By contrast in fields with areas of poor drainage water accumulates for long periods of time on those areas. This creates high humidity conditions that favour late blight development.
• Improve drainage - Consider ways to eliminate standing water in low spots or areas near the center pivot point.
• Match soil characteristics with variety growth - Varieties that develop big canopies should be planted in sandy soils.
• Avoid planting potatoes back-to-back - Do not plant potatoes in the same field for 2 consecutive growing seasons. Back-to back growing makes control of volunteers impossible.

Seed Selection and Seed Handling

Seed transmission is an important means of starting late blight infections in fields and/or production areas.

• Use certified seed - Although certified seed might not be completely late-blight free, the fields have been scouted carefully. The risk of late blight infection is less than that of using year-out seed.
• Do not mix seed lots - Keep seed lots separated to avoid mixing healthy lots with potentially infected seed lots.
• Grade seed carefully - Eliminate any tubers with a firm, reddish-brown decay. They are probably infected with late blight.
• Disinfect cutting knives between seed lots -
  Always clean and disinfect cutting equipment before cutting a new seed lot.
• Use a seed treatment labeled for late blight - Some seed treatments contain mancozeb, a fungicide that provides protection against seed-borne blight. Mancozeb will not cure existing infections, but should protect healthy tubers from being infected. Seed treated with mancozeb should be dry, not sweating.
  

At Hilling

• Form hills that cover the developing tubers with sufficient soil to minimize tuber infection -
  During irrigation or rainfall, spores are washed down from foliage and can easily infect tubers that are near soil cracks.
• Achieve good weed control - Weeds interfere with fungicide coverage leaving potato plants without fungicide protection. To obtain weed control in dry springs, you need to irrigate after spraying in order to activate the herbicide.
  

Early Season

• Make sure there are no cull piles in your farm -
  Late blight is a community disease. A single cull pile can cause an epidemic with serious economic losses for many growers. Monitor sites where cull potatoes were buried, and eliminate any volunteer potatoes.
• Start your fungicide program early - Begin a fungicide program early, when plants are 12-15 cm tall. Alternate fungicides from different chemical groups to delay the development of resistance.
  Spray cymoxanil (Curzate) at 80% emergence if the field is at high risk of late blight.
• Keep new growth protected - New growth is a good target for late blight, especially the growing point where water persists for longer periods after a rain.
  
• Try to achieve good spray coverage - Calibrate your sprayer and use a volume of water that will ensure even coverage of the canopy.
• Start scouting fields early - Scouting should start shortly after crop emergence. Risky areas that should always be monitored are:
  • low-lying areas that tend to be wet for long periods after rainfall
  • compacted areas
  • rows close to tree lines
  • field edges along creeks or ponds
  • pivot center points and pivot wheel tracks. Look closely at the plants under the first tower of center pivots. This area remains wet for long periods than farther out on the boom.
  • weedy areas
  • windward sides of fields. Spores that blow into the field infect these sides first.
  • areas protected from the wind where leaves remain wet longer after a rain.
    
    

  Walk the wheel tracks of center pivots because these tracks usually remain damp or wet after other parts of the field have dried.

• Monitor last years' potato fields for volunteers - Volunteer potato plants are common where soil temperatures do not reach -4 C to -6 C four to six inches deep during the winter. Use labeled broadleaf herbicides where possible to suppress growth of volunteer potatoes in rotational crops.

• Destroy hot spots - If late blight is found in a localized spot, destroy all diseased plants plus a border of surrounding plants. You can pull and bag the diseased plants, spray them with a herbicide, or disk the area. If you decide to disk the hot spot, spray the crop with a fungicide before disking to avoid spreading spores on farm equipment while driving out of the field. Pressure wash the equipment when finished.

Mid-Season

• Scout your fields - Keep scouting fields regularly. By midseason, rows are closed and the lower part of the plants remains wet for long periods after a rain. This allows late blight spores to germinate and initiate infections. Pick plants at random in risky areas, and check the base of the stems for blight. Particular attention to should be given to scouting after a cool, rainy period.
• Check for alternate hosts - Hairy nightshade is a host of late blight. This weed is becoming more common in Ontario, and may be found growing at the edges of potato fields. Infected nightshades spread spores to neighboring potato fields. Destroy this weed as soon as you find it.
• Manage irrigation - Try to irrigate late at night or early in the morning so that foliage can dry quickly during daytime hours.
• Continue the fungicide program alternating fungicides of different chemical families.
• Check the spray coverage - Make sure the spray coverage is even on the canopy. Before spraying, put paper strips sensitive to water in the canopy to evaluate spray coverage in the field.
• Destroy hot spots - If late blight is found in a localized spot, top kill the spot plus a 2-metre border of surrounding plants. These plants might be infected, but show no symptoms of late blight. It takes 3 to 4 days for symptoms to develop after infection.

Late Season

• Keep scouting your fields - Particular attention should be given to scouting after a cool, rainy weather. This is important for detection of late season infections.
• Kill hot spots or field - Rain washes spores down cracks in the soil into the tuber zone. If heavy rain is forecast, consider top killing infected fields. This will reduce the risk of tuber infection.
• Top kill vines at least 2 weeks before harvest - The vines should be completely dead before digging to reduce the risk of tuber infection. Green vines may be releasing spores of late blight that can infect tubers at harvest
  

Harvest

• Determine the level of tuber infection in the field prior to harvest - If there was late blight in the field, dig samples of tubers from the affected areas before digging the whole field. Harvest areas where you found 1% to 2% tuber infection separately. Ship these tubers as soon as possible. In fields that looked healthy during the season, do dig tests at random. Make sure to include risky areas of fields.
• Harvest when vines are completely dead - Make sure that vines are completely dead before harvest. The late blight fungus does not survive in dead vines.
  Harvest when skin is set. Minimizing skinning, cuts and bruises will reduce the likelihood of tuber infection. Although late blight does not need a wound to infect tubers, infection is more likely to occur in damaged areas. There is no skin to protect the tuber, and the area remains wet for an extended period.
• Avoid harvesting during wet conditions.
  

Storage

• Grade out suspicious tubers - If tuber infection is about 2%, your best option is to market the crop immediately. If this is not possible, store the potatoes near the door to move them easily and quickly.
• Remove vines, soil clogs and anything else that may interfere with air distribution in the pile.
• Dry out wet tubers - If late blight was present in the field, it is very important to dry the wet tubers quickly.
  • Provide high volumes of airflow throughout the pile. This is critical during the early storage period.
  • It may be necessary to run the fans continuously with reduced or no humidity until tubers are completely dry.
  • Market the potatoes early otherwise there will be increased pressure bruising and shrinkage losses in potatoes stored under low humidity.
    
    
• Monitor storage daily - Bad odour and wet spots indicate storage problems. Examine diseased tubers. A dry, granular light brown rot under the skin is the typical symptom of late blight in tubers. If hot spots develop, supply additional air to those areas and try to remove the potatoes as soon as possible. Holding potatoes at temperatures below 7°C may reduce development of late blight in storage, but this can have a significant impact on the market use of potatoes.

Best results are obtained by timely fungicide applications made before disease appears. Once late blight develops in a field, it will not go away.

Notes on Fungicide Applications

• Timing of fungicide applications - For a successful late blight management program, the crop must be protected with a fungicide - effective on late blight - before late blight spores are blown into the field.
• Spray coverage - Spray coverage should be even, with no skips or areas untreated.
• Spray intervals - They should be as short as 3 or 4 days if the weather is conducive to late blight and disease pressure is high. Potato growth is fast when the weather is favorable for late blight. If warm, dry weather prevails, intervals can be extended according to crop growth.
• Fungicides registered for late blight - Consult OMAFRA Publication 363, Vegetable Production Recommendations, for the list of fungicides registered to control late blight.

Consult OMAFRA publication 823, Potato Field Guide, Insects, Diseases and Defects for late blight identification.