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Department of Agriculture, Aquaculture and Fisheries
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| Potato Late Blight Recommendations for 2000 |
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Traces of late blight were present in many fields in 1999. Producers should grade their seed before planting and begin their spray program as soon after 100% emergence as possible. Monitor fields throughout the season and pay particular attention to stem infections. Stem infections are not destroyed during dry periods and can re-activate in humid weather.
Good management includes sanitation, cultural practices, field monitoring and an effective fungicide spray program.
Sanitation, Cultural Practices and Field Monitoring
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Inspect seed potatoes within 48 hours of delivery. Cut a sample of tubers and look for the reddish, brown, dry rot characteristic of late blight tuber rot. The buyer may request a re-inspection of seed potatoes within 48 hours of delivery.
- Grade seed potatoes before planting. It is important that seed is graded after it is cut and any late blight tuber rot removed before planting. Infected tubers, under wet conditions in the spring, can be a source of early infections.
- Bury cull piles before emergence of the crop. Infected tubers in cull piles are major sources of infections. Some buried tubers may germinate and grow. Rogue or treat these plants with a herbicide. Slivers and pieces of potato remaining from cutting operations should also be buried.
- Volunteer potato plants can be sources of infection even if they are growing in a cover crop such as a cereal. If there are large numbers of volunteer potato plants in a field an effort should be made to remove these plants by roguing or herbicide treatment. In fields not entered for seed certification and where late blight is prevalent, a producer might consider applying a sprout inhibitor to control volunteers the following year.
- If late blight is identified in a field, roguers and other workers should wear pants and boots which can be hosed with water or disinfected with bleach diluted 1:9 with water between fields or farms. Equipment should be washed or disinfected.
- Construction of a good hill over the potatoes will help prevent spores from washing through the soil and infecting the tubers.
- Monitor your crops. Check places in fields where moisture persists after rains or dews such as low areas and along the edges of woodlots. Have a good look at the stems and leaves of the plants for symptoms of late blight.
- When late blight is first identified destroy infected plants to reduce early sources of infection. If infected plants are rogued they should be placed in bags when walking through fields. Top kill or rogue an area twice the size of the area with infected plants.
- Rolling or rotobeating a crop before top killing exposes the soil and lower canopy to drying. Rolling also seals cracks in the soil and reduces the potential for tuber infections
- Top kill at least 2 weeks prior to harvest to allow time for infected tubers to rot and to promote tuber maturity and thicker skins at harvest. Vines should be completely dead at harvest.
- Spores survive longer in wet soils. Harvest when the soil surface is dry or windrow the potatoes to allow the surface of tubers to dry before harvest.
- Dig sprayer rows and low areas last and store these potatoes where they can be easily moved in case of problems.
- Wet or bruised tubers are more likely to be infected with late blight. Minimize skinning, cuts, bruises which provide direct entry points for late blight as well as other diseases
- Grade potatoes before they are put into storage.
- If late blight is seen on the foliage or stems there is a good possibility that there will be some tuber infections. These crops should be ventilated with a high volume of air at low humidity until the potatoes are dry. This may mean higher shrinkage than normal but losses due to storage rots will be controlled.
- Potatoes with 5% or more tuber infections are very difficult to store. These potatoes should be stored in the front of the storage or in separate bins so they can be easily removed if problems develop.
Fungicide spray program
A preventive spray program is recommended for 2000. Systemic fungicides are used in a preventive program as part of an integrated pest management (IPM) program to control resistance. Good control by fungicides requires good coverage of the leaves, proper rates of application and proper timing of applications.
In a preventive program the first 3 sprays are the most important sprays of the entire season.
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Begin spraying as soon after 100% emergence as possible but no later than the 1st week of July. Producers may use the lower rates listed on the label if the sprays are applied as a band over the row. Otherwise the full rate is recommended.
- Let the spray booms fill and run for a minute at the edge of the field before starting to spray the crop. This will avoid spraying water or low concentrations at the corners of the field.
- Starting at the opposite end of the spray row and spraying in the opposite direction each time a field is sprayed will provide better overall coverage of a crop. This is especially true for a variety such as Shepody which has cupped leaves and it is difficult to get even coverage over the whole leaf.
- Application volume should be at least 233 l/ha (52 gal/ha or 21 gal/acre) applied at 690 kPa (100 psi). Select a nozzle that produces a droplet spectrum between medium and fine.
- The spray intervals during the season should be 5 - 10 days depending on the late blight forecast. Producers should consider the shortest interval if 20 - 25 mm or more of rain occurs in a 24 hour period and during active growth of the plants (July).
- The application of fungicides should continue after top killing until the plants are completely dead.
- Copper fungicides can be applied with the top killer or after top killing. Copper on the soil will kill spores that wash off the leaves and stems and fall to the ground. These spores can cause tuber rot late in the season.
- If an area of a field is to be top killed because of late blight infections, spray the whole field with one of Tattoo C, Curzate 60 DF or Acrobat MZ. Spray the infected area last and exit the field. Then top kill the infected area using diquat or gramoxone. Spray the infected area with a copper fungicide 2-3 days after topkilling.
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