Tomato Late Blight

Tomato (Lycopersicon esculentum) -- Late Blight

Hosts: Many plants in the Solanaceae family are susceptible to late blight, including tomato, potato, pepper, and eggplant.

Cause: Phytophthora infestans, a fungus-like microorganism that usually does not survive in soil or dead plant debris.

For an epidemic to begin in any one area, the microorganism must overwinter in potato tubers (culls, volunteers) or be reintroduced on seed potatoes or tomato transplants, or live spores must blow in with rain.

Besides potato and tomato, P. infestans can infect only a few other, closely related plants. Occasionally peppers and eggplants are mildly infected, as are a few related weeds such as hairy nightshade (Solanum sarrachoides) and bittersweet nightshade (Solanum ducamara).

It is a common problem west of the Cascades. Since 1990, there have been severe outbreaks of late blight in commercial and home garden plantings of potato and tomato in both the United States and Canada. Many have been associated with new strains of the late-blight microbe.

Late blight of tomato is a disease that progresses very rapidly. Cool, wet weather favors disease development; hot, dry weather checks it. Infected stems can harbor the pathogen in dry weather, enabling sporulation and late-blight spread when wetter conditions return. Rainy weather, fog, and dew are favorable for late blight.

No commercial cultivar has sufficient resistance to P. infestans to slow disease development, and once disease begins, cultural controls may fail to slow its development. Chemical control, initiated before disease begins, is the only method that can prevent complete crop destruction.

Symptoms: Lesions start as irregular, greenish, water-soaked spots on leaves, petioles, and/or stems. Under cool, moist conditions, spots rapidly enlarge to form purplish black lesions. Lesions can girdle affected stems, killing foliage farther out. During periods of high humidity and leaf wetness, a cottony white mold usually is visible on lower leaf surfaces at the edges of lesions. In dry weather, infected leaf tissues quickly dry up, and the white mold disappears. On green fruit, gray-green water-soaked spots form, enlarge, coalesce, and darken, resulting in large, firm, brown, leathery-appearing lesions. If conditions remain moist, abundant white mold will develop on the lesions, and secondary soft-rot bacteria may follow, resulting in a slimy wet rot of the entire fruit. On ripe fruit, lesions have cream-colored concentric zones which eventually coalesce and affect the entire fruit.

Close-up of a leaf with late blight. The whitish fungal growth is characteristic. Look for it early-morning while the relative humidity is high.

Cultivar Resistance: The cultivar 'Legend' has late blight resistance in OSU-trials. WSU researchers have shown that 'Legend' may not be resistant to late blight under high disease pressure. Some cherry tomato cultivars ('Red Cherry' and 'Sweetie') were more tolerant to late blight in WSU trials. 'Matt's Wild Cherry' was considered resistant.

Cultural control: Cultural controls alone won't prevent disease during seasons with wet, cool weather. However, the following measures will improve your chances of raising a successful crop.

1. Plant only healthy-appearing tomato transplants. Check to make sure plants are free of dark lesions on leaves or stems. If starting transplants from seed, air-dry freshly harvested seed at least 3 days.
2. Destroy volunteer tomatoes and potatoes routinely by cultivation or herbicides. Do not let volunteers grow, even on compost piles. Infected tomato refuse should be buried or bagged and put in the trash.
3. Avoid wetting foliage when irrigating, especially in late afternoon and evening.
4. Space, stake, and prune tomato plants to provide good air circulation.

Management in Organic Production Control of tomato late blight is dependent upon the combination of cultural management practices and limited fungicide use.

It is imperative to use disease-free transplants. Growing tomatoes under plastic or in glasshouses will reduce the environmental conditions required for infection, namely moisture on leaves. Keeping greenhouse temperatures warm and providing good air circulation will make the plants less conducive to late blight infections.

Consider row orientation in larger plantings in the field. Select fields where winds enhance drying of the plants or have shorter dew periods. Apply irrigation water to the soil, not the foliage. Because late blight can overwinter on volunteer potato or tomato as well weedy solanaceous plants, it is imperative that volunteers and nightshades be controlled in organic production.

Discovery of late blight nearby or within a planting would call for protective spray measures as well as removal and destruction of infected plants or plant parts. Application of fixed copper with an acceptable labeling for organic use can offer protection against the oomycete that causes late blight, however; copper accumulates in soil. The biocontrol material, Serenade, suppresses late blight and in rotation with copper resulted in marketable fruit even in the presence of active late blight pressure.

Rotating out of solanaceous hosts for three years will help to avoid copper build-up as well as populations of other pathogens that affect tomato.

Interplanting tomatoes with other crops may aid in decreasing spread once an epidemic commences.

Chemical control: Spray at regular intervals. Begin chemical control programs before symptoms appear.

1. Agri-Fos at 1.25 quarts/A in 90 gal water to 2.5 quarts/A in 200 gal water. First application at transplant or when direct seeded crops are at 2 to 4 true leaf stage, then at one- or two-week intervals as required to control disease. 4-hr reentry.
2. Alude at 1.25 to 2 quart/A on 7- to 14-day intervals. 4-hr reentry.
3. Bordeaux 8-8-100 offers limited control.
4. Bravo Ultrex at 1.3 to 1.8 lb/A on 7- to 10-day intervals. 12-hr reentry.
5. Cabrio EG at 8 to 16 oz/A or at 8 to 16 oz/100 gal spay volume (dilute). Begin applying before disease develops, then follow with a labeled non-strobilurin fungicide 5 to 7 days later. Do not make more than one (1) applications of Cabrio EG or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 0 days. 12-hr reentry.
6. Cuprofix Ultra 40D at 1.25 to 3 lb/A on 5- to 10-day intervals. 12-hr reentry.
7. Dithane DF Rainshield at 1.5 to 2 lb/A or Dithane F-45 Rainshield at 1.2 to 1.6 quarts/A. Do not apply within 5 days of harvest. 24-hr reentry.
8. Echo 720 at 1.375 to 2 pints/A on 7- to 10-day intervals. Preharvest interval is 0 days. 12-hr reentry.
9. Evito 480 SC at 5.7 fl oz/A on 7-day intervals. Do not make more than one (1) application of Evito 480 SC or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 3 days. 12-hr reentry.
10. Flint (Group 11 fungicide) at 4 oz/A on 7- to 10-day intervals. Must be tank-mixed and alternated with another protectant fungicide. Do not apply within 3 days of harvest. 12-hr reentry.
11. Forum at 6 fl oz/A (non-staked plants) must be applied as a tank-mix with another fungicide that has a different mode of action (non-group 15) on a 5- to 10-day schedule. Do not make more than two (2) sequential applications of Forum or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Do not apply within 4 days of harvest. 12-hour reentry.
12. Kocide 2000 at 1.5 to 3 lb/A or Kocide 3000 at 0.75 to 1.75 lb/A on 5- to 10-day intervals. 24-hr reentry.
13. Kop-R-Spray at 1 to 3 teaspoons/gal.
14. Maneb 75 DF at 1.5 to 2 lb/A. Do not apply within 5 days of harvest. 24-hr reentry.
15. ManKocide at 2.5 to 5 lb/A on 3- to 7-day intervals. Do not apply within 5 days of harvest. 24-hr reentry.
16. Nu Cop 50 WP at 2 to 3 lb/A on 7- to 10-day intervals. Do not apply within 1 day of harvest. 24-hr reentry.
17. Quadris at 6 fl oz/A on a 5- to 7-day interval. Do not apply until 21 days after transplanting or 35 days after seeding. Do not apply with an adjuvant due to possible phytotoxicity. Do not apply more than one (1) foliar application of Quadris or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. May be applied the day of harvest. 4-hr reentry.
18. Quadris Opti at 1.6 pints/A on 5- to 7-day intervals. Do not apply until 21 days after transplanting or 35 days after seeding. Adjuvants should not be used as they may increase the potential for severe phytotoxicity. Do not make more than one (1) foliar application of Quadris Opti or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Quadris Opti should not be applied within +/- 6 days of a postemergence broadcast application of Sencor. Preharvest interval is 0 days. 12-hr reentry.
19. Ranman at 2.1 to 2.75 fl oz/A on 7- to 10-day intervals. Do not make more than three (3) applications of Ranman (Group 21 fungicide) before alternating for three intervals to a labeled fungicide with a different mode of action. Preharvest interval is 0 days. 12-hr reentry.
20. Reason 500 SC at 5.5 to 8.2 fl oz/A on 5- to 10-day intervals. Do not make more than one (1) application of Reason 500 SC or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action Do not apply within 14 days of harvest. 12-hr reentry.
21. Revus Top at 5.5 to 7 fl oz/A on 7- to 10-day intervals. Do not make more than two (2) foliar applications of Revus Top before alternating to a labeled fungicide with a different mode of action. Do not use on varieties in which mature tomatoes will be less than 2 inches. Preharvest interval is 1 day. 12-hr reentry. 17. Ridomil Gold Bravo SC at 2.5 pints/A. Use in a protectant fungicide program. Do not apply within 5 days of harvest. 48-hr reentry.
22. Tanos at 8 oz/A starting prior to disease development on 5- to 7-day intervals. Must be tank-mixed with an appropriate contact fungicide with a different mode of action (e.g. mancozeb, chlorothalonil etc.). Do not apply within 3 days of harvest. 12-hr reentry.
23. Topaz at 1 to 3 quarts/A in 100 gal water on 2- to 3-week intervals for foliar application. 4-hr reentry.

Biological control:
1. Serenade WP at 4 to 8 lb/A on 5- to 7-day intervals. Applications can be made up to harvest. Can rotate with other fungicides. 4-hr reentry.
2. Sonata at 2 to 4 quarts/A on 7- to 14-day intervals. Can be applied up to and on the day of harvest. 4-hr reentry.

References:
Shoemaker, PB, DC Milks, and NP Lynch. 2003. Fungicides and combinations for tomato late blight. F&N Tests 58:V101 Content edited by: Cynthia M. Ocamb on January 1, 2008