Late Tomato Blight

Late blight was confirmed on tomatoes in Beaver and Somerset Counties in early June. That means that farmers and gardeners should be spraying fungicides on these crops to prevent infection. Keep in mind that fungicides prevent infection they do not cure it. All plant surfaces should be covered with fungicide before conditions favor infection. Do not wait until after the rain stops to apply fungicides – they should be sprayed and dry before rain starts. Thorough coverage of all plant surfaces is needed for good control.

Late blight is a disease that can kill plants and wipe out crops in a few days if conditions favor disease growth. Those conditions described the 2009 growing season – cool to moderate temperatures with wet weather. Once you have seen late blight wipe out a garden or field, you begin to understand how a plant disease could have changed the course of history.

Late blight affects primarily potatoes and tomatoes, not other garden crops. Spores can blow for many miles, land on a wet tomato or potato leaf and start a new infection. So the disease can quickly spread from one garden to another. Infections can be prevented by regular spraying with fungicides containing chlorothalonil or mancozeb, but applications must begin before the disease gets started and continue through the growing season every 7 days to protect new growth. If weather is dry you can stretch this interval, but if we have heavy rains you may have to shorten it.

Copper based fungicides can also be used but are not as effective so must be sprayed even more often. Copper based fungicides are the only materials accepted by organic growers that have been shown to give effective control.

I wish I did not have to be the bearer of this bad news, but at least this year gardeners have some forewarning and a chance to protect plants from late blight.

Home Garden Late Blight fungicide recommendations:
Spray protectant fungicides at least every 7 days as long as plants are producing.

• Active ingredients: Chlorothalonil Daconil
• Trade Names:
• Fung-onil
• Ortho Max Vegetable Disease Control
• Possibly other trade names not listed
• Mancozeb Mancozeb flowable
• Manzate 200
• Copper Sulfate
• Copper Hydroxide
• And other copper based products – many trade names, not all approved for organic production

Chlorothalonil preferred once harvest begins because it can be sprayed up to day
of harvest. Mancozeb cannot be sprayed within 7 days of harvest – check label.
Fungicides are protective – they do not cure established infections. They are effective when used before the disease is widespread on the plant or in the garden and when all plant surfaces are covered. Also they need to be on the plant and dry before conditions favor disease germination – before rain wets the leaves.

Use all pesticides strictly according to label directions.