Scouting and Identifying Tomato Diseases

- [Marley] Disease scouting and identification is the first step to a successful disease management plan.

In this video, we'll review how to properly scout and what to look for in your tomato crop.

Early blight, Septoria, bacterial diseases, and late blight can all cause serious problems in tomato plantings.

Tomato fields should be scouted throughout July and August.

In addition to scouting, it can be helpful to pay attention to websites such as TomCast, which predicts when conditions are ripe for diseases like early blight and Septoria, or Blitecast, for late-blight forecasting.

In order to determine if you have a problem before it gets out of control, once a week, randomly select 10 plants evenly distributed in the field for close examination.

Keep trouble spots in mind, such as wet areas and field edges where problems typically develop first.

But be sure not to focus only on those areas.

Examine the upper and lower leaves on each of the 10 plants.

Make sure to look at both the upper leaf surface and the underside of the leaves.

For each plant, examine five leaves throughout the plant canopy for disease symptoms.

Make sure to look at the leaves close to the ground, in the middle of the plant, and in the top of the plant canopy.

Note which diseases are present and what percent of the leaf tissue is affected.

It is important to scout each cultivar separately and note which ones are affected most.

In some cases, you will also need to look at the roots in order to determine what is happening to your plant.

Let's look at a few common problems and their symptoms.

Early blight symptoms are easy to distinguish because they have a bullseye shape with concentric rings.

It will appear as dark brown irregular-shaped rings with lighter coloring in between.

Often, there is yellow haloing around the outside of the lesion.

Stem lesions can occur at any stage.

Initially they are small, dark, and slightly sunken, and then enlarge and form concentric rings.

Fruit can become dark with leathery, sunken spots, usually at the point of stem attachment, and can involve the entire upper portion of fruit.

Fruit can be infected at the green or ripe stage through growth cracks and wounds and often drops from the plant before mature.

Foliar symptoms of bacterial spot include black spots on the leaves which often have a yellow halo.

Lesions will look similar on the underside of the leaf.

The spots often coalesce, forming larger areas of dead tissue on the leaves.

Bacterial speck and spot are more clearly differentiated by symptom development on the fruit.

The bacterial spot will have larger lesions that can have a corky appearance or texture, as pictured here.

Bacterial speck lesions are only slightly raised and are generally much smaller, about 1/16th of an inch, than those of bacterial spot.

Speck lesions are very superficial and do not crack or become scaly as in bacterial spot.

Bacterial spot or speck can also be easily confused with Septoria.

Both often start on the lower leaves.

Septoria can defoliate tomatoes, often starting from the ground and working its way up the plant.

Heavily infected leaves will turn yellow, dry up, and drop off.

This defoliation will result in sun scalding of the fruit.

Small water-soaked circular spots of about 1/16th to one eighth of an inch in diameter first appear on the undersides of older leaves.

The centers of the spots are gray or tan and have a dark brown margin.

As the spots mature, they enlarge to about one quarter of an inch in diameter, and may coalesce.

In the center of the spots are many dark brown, pimple-like structures which are the fruiting bodies of the fungus.

Spots may also appear on stems, calluses, and blossoms, but rarely on fruit.

Late blight can develop on all parts of a plant, and can kill tomatoes and potatoes in as quickly as 14 days.

On leaves, pale green to brown spots appear first on the upper surfaces.

Spots often appear towards the top of the plant canopy from spores moving in on wind currents and thunderstorms but may also be found throughout the canopy.

Lesions can appear greasy, especially when wet.

In moist conditions, a downy, white growth usually develops near the margins of the leaf on the underside of the leaves.

The best time to see these white spores is early in the morning when there's still dew on the leaves.

If you are unsure, you can incubate some leaves overnight on your countertop in a Tupperware with a damp paper towel to see if spores develop.

Late blight causes fruit to take on a leathery appearance, generally close to where the fruit meets the plant stem.

These lesions will be somewhat sunken and have indefinite margins.

Late blight stem lesions are similar to leaf symptoms, appearing initially as greasy or water-soaked marks that quickly turn brown and dry.

These lesions may girdle the plant.

Timber rot or white mold typically initiates in a leaf axle where a flower blossom falls and becomes lodged or on damaged stems or petioles.

The pathogen does not infect healthy tissue until after it is colonized, dead, or senescent plant parts such as flower or leaves.

The lesions initially have a water-soaked appearance and will gradually enlarge to cover the stem.

Older lesions will eventually become bleached and light gray to brown in color.

When environmental conditions favor the pathogen, dense white mycelium as seen here will develop on the surface of the lesions.

And large black sclerotia will develop on the outer surface of the lesions or within the stem.

Leaf mold and gray mold are often a problem in greenhouses and high tunnels, where air flow is limited and the humidity is high.

The most readily apparent symptoms are yellow lesions on the upper leaf surfaces.

Olive green to brown sporulation is characteristic of leaf mold on the underside of tomato leaves, opposite the yellow lesions.

In certain conditions, severe defoliation can occur on susceptible varieties.

Remember to keep in mind that you may not always be able to tell what the problem is by sight.

Be sure to ask your local extension educator or send a sample to the Penn State Plant Diagnostic Lab to confirm what the problem is if you are unfamiliar.