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Plant Pathology Infobytes

August 14, 1997

HELP! What Killed My Tomato Plants?

County Extension offices receive more calls concerning tomato problems than for any other home garden crop. The 1997 summer crop was especially hard hit, since diseases caused problems for many home gardeners. All-in-all, it's been a bad year for tomatoes.

Gardeners who experienced diseases which killed their tomato plants during the summer crop and plan a fall planting should try to have the "killer" identified. This will insure use of the appropriate control measures to keep those plants alive to a ripe old age in the fall garden.

The following descriptions of symptoms caused by the various tomato diseases should help you identify the killer.

Fusarium wilt is one of the most common tomato killers which may show up on plants any time from blossoming to fruit maturation. Look for the following symptoms to identify this disease.

  • Yellowing of older (lower) leaves, especially on one side of the plant is one of the first symptoms.
  • Plants wilt during hottest part of the day but recover from the wilt late in the afternoon.
  • The wilting becomes more extensive from day-to-day, and finally the plant dies.
  • You can tell if you have Fusarium wilt by looking for the key symptom, which is a zone of dark brown discoloration of stem tissue at the point just above the soil line. In this area, make a shallow (about 1/4 inch deep) cut and peel back the tissue and check for the discoloration. The browning of stem tissues is generally visible up the stem for 18 to 24 inches.

Southern stem blight is also a common disease which shows up later in the season, particularly following several days of wet and hot weather. This disease is easy to identify.

  • Infected plants wilt and die very quickly.
  • Southern stem blight causes brown discoloration of lower stem tissues which may be confused with symptoms of Fusarium wilt; however there's an easy way to distinguish southern blight from Fusarium wilt. Look for the key symptom, which consists of white web-like fungus growth near the soil line.
  • Further evidence of this disease is the presence of small, creamy white to brown seed-like structures about the size of mustard seeds embedded within the web of fungus growth (these are the reproductive structures of the southern blight fungus).

Bacterial wilt is a common tomato plant killer in the southern part of the state. This disease is more difficult to identify, but a close check for a few give-away symptoms will help identify this disease.

  • Bacterial wilt affected plants often look perfectly healthy in the morning but by mid-afternoon, look as though they have had hot water poured on them.
  • Look for extensive rotting of the pith in the lowest part of the stem and upper root system --- generally the pith tissue is rotted away, and remaining tissue is yellowish to orange colored.
  • A section of stem suspended in a container of water exhibits "bacterial streaming." For a description of this technique check by your county Extension office County Extension Office and ask for "Plant Disease Dispatch on Bacterial Wilt of Tomatoes."

Root-knot nematodes sometimes cause death of susceptible tomato varieties, but more often, these nematodes are associated with stunted growth.

Several other problems which may cause death of tomato plants are not related to fungus or bacterial diseases. One of the most common is the "if a little bit is great, then a lot is going to be even better" practice of fertilizing tomatoes and other garden plants with ammonium nitrate and other fertilizer materials. Over fertilization leads to burning of roots, irreversible plant wilting, and frequent plant death. Initial symptoms of fertilizer burn often show up on leaves, which may turn brown and appear to be "fried." The best way to check for fertilizer burn is to collect a soil sample (about one pint) from around the plant for analysis.

One of the most common causes of tomato plant death in the 1997 season was extensive early season rainfall. During extremely wet periods in poorly drained garden sites, plants frequently wilted as a result of soils which remain saturated for several days. Wilting results from oxygen starvation of the roots. "Water wilt" can be confirmed by removing a plant from the soil. Dark, sour smelling roots are a good indication that poor drainage is responsible for the problem. Also, the area where the plant was removed may contain standing water, adding further evidence. Provided water drains away from roots within a day or so, "water-wilted" tomato plants sometimes recover.

If any of these "tomato plant killers" were at work in your garden this season, check by your county Extension office for assistance in avoiding a repeat of the problem in the fall crop.

Infobytes newsletter was written by the late Dr. Frank Killebrew, Extension Specialist.