CORVALLIS - What can a home gardener do to discourage late blight on potatoes and tomatoes? Oregon State University plant pathologists Cindy Ocamb and Mary Powelson recommend the following steps:
- Buy only healthy-appearing tomato transplants, free of dark lesions on leaves or stems. Plant only healthy-appearing potato seed pieces.
- Destroy volunteer tomatoes and potatoes in your garden, as they may harbor the disease. Remove and bury, or bag and discard, all tomato and potato refuse laying around.
- When planting, space tomato transplants and potato seed pieces far apart. Stake and prune tomatoes to provide good air circulation.
- Check for the disease regularly, in the early morning, when it shows up best.
- Irrigate your plants in the morning rather than in the afternoon or evening. Avoid wetting the leaves when irrigating - drip irrigation is better than overhead sprinkling.
If you want to be absolutely sure that you won't lose your tomatoes or potatoes to late blight this summer, Powelson and Ocamb recommend applying protective fungicides now, before you see any symptoms.
Fungicides act as a barrier to the disease, explained Powelson. But they do not cure a plant once it is diseased.
"Once you see any symptoms of this disease, it is frequently too late to effectively control it," said Ocamb. "Prevention is the name of the game."
Products containing chlorothalonil are available to the homeowner and are quite effective if used properly. Ortho's Multi-purpose Fungicide, Daconil is one such product and is widely available. Use one tablespoon per gallon of water. As with all pesticides, always read the label for instructions on proper handling and application.
Products containing Maneb are also available for late blight control by the home gardener. Hi-yield Maneb Lawn and Garden Fungicide is available in some garden supply shops. Use 1.5 tablespoons per gallon of water.
For gardeners wishing to not use synthetic fungicides, there are copper products such as Hi-Yield Copper Fungicide, Kocide, or Lilly Miller Microcop Fungicide. These are rated as moderately effective on late blight. See the product label for concentrations as each of these products directions varies.
"Cool, wet weather favors late blight development," said Ocamb. "Hot, dry weather checks it. Infected stems harbor the fungus in dry weather, allowing the disease to spread when damp conditions return."
Powelson recommends making up a small batch in a quart spray bottle, and applying it to all your tomato and potato plants in the early morning every two weeks when it is warm and dry.
If the weather turns wet and cool, apply sprays every 5-7 days. In August, when the morning fogs return, it is especially important to spray the fungicide regularly, she said. Make sure to completely cover each plant with the chemical as a non-sprayed area is unprotected thus, vulnerable to late blight infections.
For more information about late blight outbreaks around the state, contact a horticulturist at local county offices of the OSU Extension Service.