Rideout: Tomato plants vulnerable to broad range of wilt diseases

It is that time of year when home gardeners start setting their tomato plants. Tomatoes are the most popular vegetable grown by gardeners but they are also susceptible to a broad range of viral, bacterial, and fungal diseases, especially when exposed to wet conditions.

Of these diseases, the most common and often difficult to diagnosis are the ‘wilt’ diseases. Properly planting your tomato’s and avoiding bad drainage areas will prevent many of these common diseases.

Fusarium Wilt is a fungal disease favored by warm soil temperatures and low soil moisture; a little different from other wilt diseases. Symptoms are first noticed as yellowing of the lower leaves, often only on one side of the plant. The yellowed leaves gradually wilt and die. As the disease progresses, successive leaves yellow, wilt, and die but remain attached to the plant.

Fusarium wilt can be confirmed by cutting the lower portion of a wilted plant’s stem lengthwise. The center of the stem is the pith. The vascular tissue is the tissue between the pith and the outside of the stem. A plant with Fusarium wilt will show brown streaking in the vascular tissue. In severely wilted plants, this brown discoloration may extend to the top of the plant.

The fungus that causes Fusarium wilt is often introduced into a garden on contaminated seed, infected transplants, wind, water, animals, or equipment. Once introduced, the fungus can survive for many years in the soil.

The only effective way to control Fusarium wilt is to use resistant varieties. Look at tomato labels or catalog descriptions for the letter F which indicates Fusarium wilt resistance.

Verticillium wilt is a fungal disease favored by cool temperatures. Unlike Fusarium wilt, Verticillium wilt will affect a tomato plant uniformly and does not show first on one side or one shoot. The lower leaves are affected first. These leaves become wilted, yellowed and necrotic, often in a V-shaped pattern.

Affected leaves eventually drop from the plant. As the disease progresses, successive leaves begin to wilt and die, until only a few healthy leaves remain at the top of the plant. The plant will be stunted and remaining fruit will develop at a smaller size.

Verticillium wilt causes discoloration of the vascular system similar to Fusarium wilt except that browning does not extend as far up the stem and is more pronounced near the soil line.

Verticillium wilt fungus can persist in the soil for many years. An infected area should be placed into a five-year rotation with grasses or corn since Verticillium wilt fungus can attack a wide range of plants, including potato, eggplant, strawberry and black raspberry.

Another way to control Verticillium wilt is to plant resistant varieties. These plants are indicated as resistant with the letter V on their label.

Bacterial wilt is a bacterial disease that affects potatoes, eggplants, peppers, and tomatoes. This disease is characterized by a sudden wilt of the plant without leaves first discoloring.

Cutting through the stem near the ground will show a darkened, water-soaked pith and vascular tissue. Later pith will turn brown and eventually become hollow.

A quick aid to diagnosis is to place a freshly cut stem piece in water. A white, milky stream of bacteria should ooze from the cut stem.

Bacterial wilt is more common in moist, warm, high pH, low-fertility soils. There are few tomatoes resistant to bacterial wilt and crop rotation is of minimal value because the bacteria can persist on many weeds and even fallow soil.

Other control options may include: avoid sites where the disease has been a past problem, remove and destroy infected plants promptly, avoid root injury, maintain proper pH and nitrogen levels, and avoid hot, wet soils.

If a tomato plant wilts but does not have the characteristics of the wilt diseases above, carefully dig up and inspect the roots for rot. A non-infectious disease called Water Wilt may be to blame.

In a well-drained soil, following a heavy rain, healthy plants use the rain for extra growth. However, tomatoes in poorly-drained soils often have roots exposed to waterlogged soil conditions for several days. This condition can lead to a slight wilting of foliage in the top of plants. If soil remains saturated, the wilt continues and the entire plant may collapse within six to twelve hours.

Tomatoes in soggy soils quickly use up gaseous soil oxygen. Plants can’t survive without oxygen and during an extended period with wet soil and unavailable oxygen, they will die.

It is unlikely that damaged plants could be dug and replanted to better draining soils with much success since the roots are frequently attacked by Pythium or Phytophthora root rot diseases. Plant replacement is your best option and if your garden site is poorly drained, it may be a good time to consider using a well-drained soil mix in a raised bed garden.

Choosing a good, resistant variety and planting in well drained soil will help ensure success this garden season. Some fungicides will help but preventing disease is always better than trying to cure disease. If you have questions about your tomato plants this season, give us a call at the Henderson County Extension Office; we are happy to help!