Tomato Disease Update

Below, I will briefly discuss a few of the diseases that I have observed on tomatoes recently.

Powdery mildew of tomato – Powdery mildew of tomato is not usually a common problem in Indiana. However, in recent years, there have been more reports of this disease than usual. Powdery mildew is more often observed in a greenhouse situation than in a field.

The key symptoms of this disease are the talc-like lesions on the upper and lower leaf surfaces (Figure 1). It is important to note that the location of the upper and lower lesions do not correspond with each other. When the lesions are young, it may almost seem as if the lesions can be ‘wiped off’.

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Figure 1. Powdery mildew of tomato.

Few varieties exist with good levels of host resistance, although growers may notice some difference in susceptibility between varieties.

It may not be necessary to treat tomatoes affected with powdery mildew with fungicides. If fungicide treatment appears to be warranted, a few alternatives are: Fontelis®, Inspire Super®, Quadris Top®, several formulations of sulfur, Switch® and Vivando®. The products listed are all either labeled for the greenhouse or silent about greenhouse use. Check the label carefully before using any of these products.

Septoria leaf spot – I have observed several fields with this foliar disease. While Septoria leaf spot was not severe in these fields, if left unchecked, the disease can cause defoliation, reduced fruit quality and yield loss.

Lesions of Septoria leaf spot often start on the lower leaves. As the disease progresses, symptoms may begin to appear higher in the plant canopy. Larger lesions may have a gray center with a dark brown margin (Figure 2). If one uses a 10X hand lens, it is possible to see dark fungal structures (known as pycnidia) in the middle of such lesions. Leaves and stems may be affected, but fruit infection is rare. Since Septoria leaf spot requires leaf wetness for infection, this disease is most often observed in the field and not in greenhouses.

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Figure 2. Septoria leaf spot of tomato.

Crop rotations of 3 to 4 years and fall tillage remain important management decisions. Currently, no commercial varieties have resistance to Septoria leaf blight. Fungicides that are likely to be effective against this disease are listed in the MW Vegetable Production Guide mwveguide.org. Contact products include fungicides with the active ingredient chlorothalonil (e.g., Bravo®, Echo®, Equus®, Initiate®) and mancozeb (e.g., Dithane®, Manzate®, Penncozeb®). Systemic fungicides include Aprovia Top®, Cabrio®, Fontelis®, Inspire Super®, Luna®, Priaxor®, Quadris®, Revus Top®, Rhyme®, Tanos® and Zing®.  See the MW Vegetable Guide for details on rates, application instructions and formulation differences. Organic growers may find that fungicides that contain the active ingredient copper may help to reduce disease severity.

Tomato pith necrosis – Often the first symptom observed is a chlorosis (yellowing) of young leaves. It might be possible to observe stems that appear shriveled. This may be associated with a dark brown necrosis of the lower stem and rotten pith (Figure 3). Low night temperatures, high nitrogen levels and high humidity favor the bacterium that causes pith necrosis. Therefore, it is usually seen in a greenhouse situation. The only management recommendation is to avoid high nitrogen levels in tomato plants as well as high humidity.

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Figure 3. Tomato pith necrosis. Note twisted stem and discoloration.

Fusarium crown and root rot – This disease is more often observed in a greenhouse situation where tomatoes are grown in the soil than the field, perhaps because of a lack of crop rotation in most greenhouses. The causal fungus, Fusarium oxysporum f.sp. radicis-lycopersici, survives very well in the soil in the absence of the host.

However, this disease was recently observed in the field in Indiana. Symptoms of this disease include tomato plants with lower leaves that become yellow (chlorotic) and die; plants that begin to wilt; a lesion on the lower stem at ground level (Figure 4). If tomato plants are removed from the soil and carefully split open from the ground level, a discoloration of the vascular tissue can be observed. It is important to note that this discoloration does not extend up the stem more than 6 to 8 inches. If the discoloration extends up into the plant canopy, the disease may be Fusarium wilt of tomato. Although growers may observe multiple plants begin to die of this disease over a period of days or even weeks, the fungus does not often spread from plant to plant. Temperatures from 68 to 72°F favor Fusarium crown. Therefore, this disease often starts in early in the growing season.

Figure 4. Stem lesion phase of Fusarium crown and root rot of tomato.

Figure 4. Stem lesion phase of Fusarium crown and root rot of tomato.

Crop rotations that do not include tomatoes or other solanaceous crops will help to lower the number of fungal spores in the soil. However, since the causal fungus survives for years without a host, crop rotation is not a complete solution. I also realize that many growers who produce tomatoes in high tunnels do not feel it is economically practical to rotate to another crop. To such growers, I would point to this article about how to minimize diseases in high tunnels. Growers who plant tomatoes in bags or pots in a high tunnel instead of in the soil should avoid Fusarium crown rot since the fungus survives in the soil.

Check with your seed representative or seed catalog for tomato varieties with resistance to Fusarium crown rot. Most tomato varieties with resistance to Fusarium crown rot are indeterminate (in contrast, there are many varieties with host resistance to Fusarium wilt.) It is possible to graft your favorite tomato variety as a scion onto a rootstock variety with resistance. This  table will help one select tomato rootstocks with resistance to Fusarium crown rot and other diseases. Some tomato seed companies will sell grafted tomatoes.

There are no fungicides to control Fusarium crown rot. Most fungicides are for foliar use; I know of no fungicides that may be sprayed on the top of the soil.  Read all fungicide labels carefully.  Contact me if you have questions.

Diseases of tomato that I have not observed yet, but expect to see soon include:

  • Bacterial spot and speck-Leaf and fruit lesions of these diseases are very common in field tomatoes.
  • Bacterial canker-This disease can cause a systemic infection of tomatoes, mostly in the field.
  • Early blight-Often observed to start on younger leaves and works its way higher in the canopy. Most often observed on field tomatoes.
  • Leaf mold-mostly observed in greenhouses tomatoes. Yellow lesions on top of leaves are often accompanied by a gray/green mold on leaf bottoms.
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