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Tomato Plant Disease & Fungus Identification

By Tammy Curry ; Updated September 21, 2017
Tomatoes ripening on the vine.
Image by Flickr.com, courtesy of Jose Oquendo

There are a range of diseases that affect tomato plants. Tomato plant diseases are caused by fungi, bacteria, nutritional deficit, viruses and nutritional deficit. The diseases attack various parts of the tomato plant. Most diseases can be identified by the portion of the plant affected, as well as physical attributes. Early identification of tomato plant diseases prevents reduced crops and crop loss.

Bacterial Diseases

Three common bacterial diseases that affect tomato plants are bacterial canker, wilt and spot. Bacterial canker is caused by Clavibacter michiganesis subsp. Michiganesis. Canker primarily affects the vascular system of the tomato plant, it is identified by the yellowing of the main stems and lower leaves. Pseudomonas solancearum causes bacterial wilt. There are no obvious early identifiers for this disease. Just prior to plant death the leaves and stems will wilt. Positive identification can be made by slicing open the stem of a plant and observing the pith (center), plants in early stages will have a watery appearance to the center, just prior to plat death the pith will appear brown and hollow. Bacterial spot is the result of infection by Xanthomonas campestris pv vesticatoria. Early stages of the disease cause spots that appear circular to irregular in shape and appear to be a darker green than the rest of the leaf. As the disease progresses the spots become enlarged turning purplish gray with black centers and the surrounding leaf tissue yellows.

Viral Diseases

Cucumber mosaic virus (CMV), Alfalfa mosaic virus and potato virus “Y” appear to affect leaves and leaf development primarily. These viruses are spread by aphids that have ingested infected plants in another field. Signs of infection are disfigured leaves that have a mottled discoloration (various shades of yellow and green). Alfalfa mosaic virus causes leaves to turn yellow, curl downward and stop growing. Eventually low production and plant death ensue. Potato virus “Y” infection can be identified by leaves that have rolled downward, have mild mottling and are misshapen.

Fungal Diseases

Early blight is caused by a specific fungus, Alternarie solani and appears as black, irregular spots surrounded by yellow leaf tissue. Fusarium oxysporum causes fusarium wilt on tomato plants. Infected leaves turn yellow, wilt and then drop off the plant, plant death occurs shortly after losing its leaves. Late blight is a fungal infection caused by Phytophthlora infestans. Large blotchy black spots appear on the leaves in the early stages. The lesions spread to the remainder of the plant and eventually any fruit that appears causing decay.

Nutritional Diseases

Soil deficiencies of nitrogen, phosphorus and magnesium primarily affect leaf development. Nitrogen deficiency can be identified when older leaves turn brown and young leaves turn yellow. Soil deficiency in phosphorus causes young plants to turn purplish and older plants to fail to produce leaves, blooms and fruit. Magnesium deficiency causes leaves to turn brown, wither and die.


Pathogens, such as bacteria and viruses, can be prevented by purchasing seeds from disease-free fields, purchasing healthy seedlings and purchasing varieties that have been bred (genetically engineered) for disease resistance. Bacterial and viral infections can be treated chemically with fixed copper bactericides. Fungal infections can be treated with fungicides. Nutritional deficiencies can be avoided by regular soil testing. Soil test results will provide you with an analysis of what nutrients need to be added to the soil.


About the Author


Currently residing in Myrtle Beach, SC, Tammy Curry began writing agricultural and frugal living articles in 2004. Her articles have appeared in the Mid-Atlantic Farm Chronicle and Country Family Magazine. Ms. Curry has also written SEO articles for textbroker.com. She holds an associate's degree in science from Jefferson College of Health Sciences.