Tomato late blight not favored by hot and dry weather

Review symptoms and recommendations to know how to manage late blight.

Late blight is a disease that most commonly affects potatoes, but can affect tomatoes in some years. Thus far, the weather has not been favorable for this disease. Late blight can be especially troublesome when the weather is cool and wet. This disease has not been reported in Michigan this year. Growers practicing IPM are not likely to see this disease since the fungicides they are using for Alternaria, Septoria and Anthracnose are also probably providing adequate protection against late blight.

Late blight symptoms include blighting on all aboveground parts of the tomato plant. Lesions on leaves often appear dark and oily with production of spores occurring on the undersides of the leaves, resulting in a purplish appearance, especially when conditions are wet and humid. Blackened lesions on the stems also occur and are unique to late blight disease.

Late blight affects green and ripened tomato fruit. The blighting on fruit appears as dark, greasy areas that enlarge rapidly, encompassing the entire fruit. During wet and humid conditions, white threads (mycelium) can be seen on the fruit.

Between cropping seasons, the fungus survives on volunteer and abandoned potato and tomato plants in fields, cull piles and homeowner gardens. Cool nights, moderately warm days and weather that keep foliage and fruit wet for several hours are ideal for late blight development. The spores can be carried from diseased plants to nearby healthy plants via wind.

Control measures include eliminating all potato and tomato cull piles in the vicinity of tomato plantings and destroying volunteer potato plants that grow from overwintered tubers. All tomato varieties are susceptible to late blight. There are several highly effective fungicide sprays that can be used to protect tomato plants from late blight.

Tomato late blight fungicide recommendations

Homeowners

  • Chlorothalonil: Apply fungicides that list chlorothalonil as the active ingredient every seven days or as the label specifies.
  • Copper: Chlorothalonil can also be mixed with copper fungicides (each at the full labeled rate).

Organic growers

Your local organic certifying agency can help you determine what products are approved.

  • Copper Hydroxide: Some forms of copper hydroxide are approved for use in organic production. Spray every 7 days or as the label specifies.
  • Bacillus subtilis: Serenade, 4-8 lb every 5-7 days (0 days PHI). Provides suppression. Biological control product that needs-be applied before disease development. Control may be limited under heavy disease pressure.
  •  Other fungicides labeled for late blight that are OMRI-listed include Sporatec, Sonata, and OxiDate.
  • NOTE: In general, products available to organic growers are not likely to halt tomato late blight when the environment favors disease (cool and wet).

Growers using integrated pest management

  • New products are marked with two asterisks (**).
  • Materials marked with an asterisk (*) are particularly recommended for problem infestations.
  • Preharvest interval is in parentheses.

** Fluopicolide: Presidio, 3-4 fl oz every 10 days (2 days).

** Mandipropamid/Difenoconazole: Revus Top, 5.5-7 fl oz every 7-10 days (1 day).

* Azoxystrobin/Chlorothalonil: Quadris Opti, 1.6 pt every 7-21 days (0 days).

* Chlorothalonil: Bravo Ultrex, 1.3-1.8 lb every 7-10 days (foliage) or 1.8-2.6 lb every 7-14 days (fruit) (0 days). BravoWeather Stik or Echo 720 or Equus 720 SST, 1⅜-2 pt every 7-10 days (foliage) or 2-2¾ pt every 7-14 days (fruit) (0 days). Echo 90DF, 1.1-1.6 lb every 7-10 days (foliage) or 1.6-2.5 lb every 7-14 days (fruit) (0 days). Equus DF, 1.3-1.8 lb every 7-10 days (foliage) or 1.8-2.6 lb every 7-14 days (fruit).

* Cyazofamid: Ranman, 2.1-2.75 fl oz every 7-10 days (5 days).

* Cymoxanil: Curzate 60DF, 3.2-5 oz every 5-7 days (3 days).

* Dimethomorph: Acrobat 50WP, 6.4 oz every 5-10 days. Forum, 6 fl oz every 5-10 days (maximum 2 applications before alternating with another mode of action) (0 days).

* Mancozeb/Zoxamide: Gavel 75DF, 1.5-2 lb every 7-10 days (5 days).

* Propamocarb: Previcur Flex, 0.7-1.5 pt every 7-10 days (5 days).

* Pyraclostrobin: Cabrio, 8-12 oz (0 days). Apply in a strict one-to-one alternation program with fungicides having a different mode of action.

Azoxystrobin: Amistar, 1.6-2 oz every 5-7 days (0 days). Quadris, 5-6.2 fl oz (0 days). Do not apply more than one foliar application of Amistar and Quadris or other strobilurin fungicide before alternating with a fungicide that has a different mode of action.

Mancozeb/Copper Hydroxide: ManKocide, 2.5-5 lb every 3-10 days (5 days).

Famoxadone/Cymoxanil: Tanos, 8 oz (3 days). Apply in a strict one-to-one alternation program with fungicides having a different mode of action.

Fenamidone: Reason 500 SC, 5.5-8.2 fl oz every 5-10 days (14 days).

Maneb: Maneb 75DF or Maneb 80WP, 1½-3 lb, every 7-10 days (5 days). Manex, 1.2-2.4 qt every 7-10 days (0 days).

Mancozeb: Dithane DF Rainshield or DithaneM-45 or Dithane WSP 1½-3 lb every 7-10 days (5 days). Manzate 75DF 1½-3 lb every 3-7 days (5 days). DithaneF-45 Rainshield, 1.2-2.4 qt every 7-10 days (5 days). Penncozeb4F, 0.6-2.4 qt every 3-7 days (5 days). Penncozeb 75DF or Penncozeb 80WP, 1½-3 lb every 3-7 days (5 days).

Mancozeb/Copper Sulfate: Cuprofix MZ Disperss, 1.75-7.25 lb every 3-10 days (5 days).

Trifloxystrobin: Flint, 4 oz every 7-10 days (3 days).

Phosphorous Acid Salts: Fosphite, 3-5 qt per 20 gal (ground) or 10 gal (aerial) of water every 14-21 days (0 days).

NOTE: While the following products may be helpful, it is likely that this strain of late blight is resistant to fungicides containing mefenoxam.

Mefenoxam/Chlorothalonil: Ridomil Gold Bravo, 2 lb every 14 days (maximum 3 applications) (14 days).

Mefenoxam/Mancozeb: Ridomil Gold MZ, 2.5 lb every 14 days (maximum 3 applications) (14 days).

Mefenoxam/Copper Hydroxide: Ridomil Gold Copper, 2 lb every 14 days (maximum 3 applications) (14 days).

For more information, see the tomato late blight fungicide trials.

Dr. Hausbeck’s work is funded in part by MSU’s AgBioResearch.

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