Late blight on Plants

Symptoms

Late blight symptoms show up on leaves, stems, fruit and tubers. Leaf symptoms are the first to appear, and the spots enlarge rapidly. They begin as water-soaked spots on leaves that rapidly enlarge into brown-black blotches. On the undersides of leaves near the spot margins, you may see a grayish-white downy mold.

Leaves then die quickly. The plant can lose most or all of its foliage within about two weeks. Stems develop dark brown to almost black lesions.

Fruit shows greasy-looking, greenish-brown spots that enlarge until the whole fruit is involved. Fruit may also have firm brown lesions, which often start on the shoulders where spores land. Both kinds of fruit damage are brown or greenish-brown.

Potato tubers show reddish-brown, dry, granular spots. A shallow, coppery-brown dry rot can also spread irregularly from the tuber surface.

Causes

Late blight is caused by infection with the oomycete pathogen Phytophthora infestans. The disease is found when humid conditions coincide with mild temperatures for prolonged periods. The key is that the humidity and the mild temperatures last. Spores spread easily by wind.

Overhead or sprinkler irrigation that wets the foliage also favors the disease, since wet leaves favor disease development. The pathogen infects tomato and potato, and it can overwinter in infected potatoes, tomatoes and nightshade weeds. Volunteer tomato and potato plants and nightshade weeds growing nearby can harbor the pathogen between seasons.

Potato cull piles and volunteer potatoes and tomatoes can carry the pathogen over winter. Crop debris in a field can also hold a winter reservoir of the pathogen. Plants bought as transplants are worth checking too, since you should reject any that show late blight symptoms.

In short, the causes cover the pathogen, the weather, the spores, the watering and the places the pathogen survives winter. Each cause on this list is something you can look for in your own garden.

Diagnosis

The facts for this page name no look-alike disease, so the checks below use only the symptoms and causes. Start with the pattern on the leaves. Look for water-soaked spots that rapidly enlarge into brown-black blotches. Then turn the leaf over and look for grayish-white downy mold on the underside near the spot margins.

Next, consider how fast the damage moves. Late blight can cause extensive to complete defoliation within about two weeks. Check the stems for dark brown to almost black lesions. Look at fruit for greasy-looking, greenish-brown spots, or for firm brown lesions that often start on the shoulders. On potatoes, check tubers for reddish-brown, dry, granular spots.

Then look at the conditions around the plant. Late blight is found when humid conditions coincide with mild temperatures for prolonged periods. Ask whether you water overhead in a way that wets the foliage. Ask whether infected potatoes, tomatoes or nightshade weeds grew nearby.

Finally, check what you grow. The hosts listed on this page are tomato and potato.

Treatments

Where late blight occurs regularly, grow tomato varieties bred to resist certain races of the pathogen. Inspect transplants carefully before planting, and reject any that show late blight symptoms. Scout your plants regularly, and cut off and bag infected leaves and stems promptly. Remove and discard entire plants that are severely infected.

Avoid overhead or sprinkler irrigation that wets the foliage, or irrigate so leaves dry quickly. Improve air circulation by eliminating weeds and removing extra branches. You can also harvest fruit early and ripen it off the vine in a warm, dry location.

Remove volunteer tomato and potato plants and nightshade weeds growing nearby, since they can harbor the pathogen between seasons. Destroy potato cull piles and prevent volunteer potatoes and tomatoes from sprouting. Disc fields in fall to eliminate the winter reservoir in crop debris.

Chemical options are protectants, so they work only when you apply them before infection. Chlorothalonil is one active ingredient listed, and mancozeb is another. Copper-based fungicides are also an option, though they are generally less effective than chlorothalonil-based products.

Other listed active ingredients include famoxadone with cymoxanil, dimethomorph, azoxystrobin, azoxystrobin with difenoconazole and pyraclostrobin. For these, ensure thorough spray coverage, and alternate between fungicides that have different modes of action. Make no more than two sequential applications before alternating with a fungicide that has a different mode of action. The product label is the legal instruction, so follow it for the crop and site you are treating.

Prevention

Prevention is about keeping conditions unfavorable for the pathogen. Avoid overhead or sprinkler irrigation that wets the foliage. If you must water that way, water mid-morning so the leaves dry quickly. Keeping leaves dry matters, since wet leaves favor disease development.

Keep air moving around plants by eliminating weeds and removing extra branches. Pull out volunteer tomato and potato plants and nearby nightshade weeds, and destroy potato cull piles. Inspect transplants before planting and reject any with symptoms.

Choose tomato varieties bred to resist certain races of the pathogen where the disease occurs regularly. Disc fields in fall to remove the winter reservoir in crop debris. Scout plants regularly, and bag infected leaves and stems promptly.

Plants most often affected

The hosts listed for late blight on this page are tomato and potato. The symptoms described cover leaves, stems, fruit and potato tubers. The pathogen can also overwinter in nightshade weeds growing nearby. Tomato varieties bred to resist certain races are available for places where the disease occurs regularly.

• Climbing Nightshade
• Garden Tomato