Organic Late Blight Management

Posted: July 31, 2012

For organic late blight management there are two important things to re-member. The first is you cannot “cure” late blight. The other is scout frequently.
late blight on tomato

late blight on tomato

Organic fungicides can only be used preventatively. If the leaf surface is covered adequately in a copper when a spore lands on the leaf it should be killed. If it has already germinated and entered the leaf, it is protected by the leaf and will continue to invade the plant.

Copper can be effective when applied preventatively. Apply every five to seven days when conditions are favorable. Thorough coverage is essential!

Keep in mind that not all coppers are created equal. They tend to work better the more Cu++ ions are released. So copper hydroxide formulations like Champ (OMRI approved last I checked) work better than copper tallates > copper ammonium > cop-per oxychloride > basic copper sulfate (Cuprofix). The copper generally sold to home gardeners is usually basic copper with a very low percent active ingredient. Talk to your local farm store for available products or order from an online retailer.

My second piece of advice is to make sure you can tell the difference between early blight, Septoria leaf spot and late blight. Then scout frequently.

Early blight and Septoria tend to develop on the lower leaves.

Early blight will first appear as irregular dark brown to black dead spots and then develop concentric rings. 

Septoria has small black dots inside the light tan colored lesion.

Late blight will have greasy gray-tan colored lesions. When it's moist, the late blight pathogen produces spores. Look for white fuzzy growth on the underside of leaves. I usually scout first thing in the morning when there is dew so I can see this. If I find something suspicious in the afternoon I put the leaf in a bag with a wet paper towel on the counter and check for fuzzy growth the next morning. Another clue is late blight tends to develop randomly on the plant versus only on the bottom leaves.

Here are suggested steps for organic late blight management from plant pathologists Meg McGrath, Cornell, and Beth Gugino, Penn State.

Steps to Managing Late Blight in Organic Systems
1. Check weather reports and with Cooperative Extension

  • Cloudy misty days favor the development of late blight
  • Check with Extension to see if there are reports of outbreaks in the area

2. Apply copper-based products preventatively

  • Every five to seven days (when conditions favor disease)
  • Good coverage is essential (under leaves too!)

3. If symptoms are found in isolated plantings

  • Immediately remove affected tissue
  • Best to remove in the middle of the day (hot/dry)
  • Put tissue in a garbage bag, under a tarp (heat will kill)
  • Inspect plants daily for one week (one time a week after)
  • Apply copper every five to seven days
  • If you are the first in your area to find it destroy a larger area, talk to Extension, get it identified! This is a community disease -- TYPHOID MARY.

4. When late blight becomes severe

  • Destroy planting or hot spot including adjacent asymptomatic plants that are probably infected, but not showing symptoms yet
  • Propane flamers can be used if there is no plastic mulch
  • In trellised tomatoes, cut all main stems and come back to cut all stems further up
  • Best in middle of a sunny, calm day
  • Next remove trellising line / stakes and flail chop
  • Bag or bury in smaller plantings
  • Do not compost infected plant material
  • Once the plant tissue is dead, the pathogen is dead!