By: Bonnie L. Grant, Certified Urban Agriculturist
Food crops are prey to numerous pest and disease issues. Diagnosing what is wrong with your plant and how to treat or prevent it can be challenging. A look at the anthracnose disease, its formative conditions, and controls can help save your tomato crop from very contagious fungal diseases.
Anthracnose is a serious disease of many crop and ornamental plants. On tomato plants, it can decimate the crop, producing inedible fruits. This is a disaster for commercial growers but also affects home gardeners. Anthracnose of tomatoes results in lesions on both green and ripe fruit. Continue reading for important tomato anthracnose info, including how to prevent and treat the disease.
What is Anthracnose on a Tomato?
Essentially, anthracnose is a fruit rot. There are many types of rot that can affect tomatoes, but anthracnose is especially prevalent. Tomatoes with anthracnose are infected with the fungi Colletotrichum phomoides, C. coccodes or several other species of Colletotrichum.
The fungus survives and even overwinters in old plant debris but can also be contained in seeds. Wet weather or splashing from irrigation provides ideal conditions for disease development, as do temperatures of 80 degrees Fahrenheit (27 C.) or more. According to tomato anthracnose info, even the harvesting of ripe fruit can dislodge infecting spores and spread the disease to otherwise healthy plants.
Anthracnose of tomatoes usually affects ripe or overripe fruits but can occasionally show up on green tomatoes. Green fruits may be infected but do not show signs until ripening. Round, sunken, water-soaked spots initially infest fruit. As the disease progresses, lesions get larger, deeper and become dark. Fruits infected with just one or two lesions are considered culls and thrown out. This is because the advanced stages of the disease penetrate deeper into the flesh causing corky, moldy spots and rotting.
It is also very contagious and removal of infected fruit can help prevent the spread of the fungus. Tomatoes with anthracnose that are contaminated by the fungus begin to show signs of lesions 5 to 6 days after contraction of the fungus.
Controlling Anthracnose of Tomatoes
Poorly drained soil promotes the formation of the disease. Crops in the Solanaceous family should be on a 3- to 4-year rotation. These would also include peppers and eggplant.
Staking or trellising plants can minimize the contact between soil borne fungi, as can applying a mulch. Watering at the base of the plants can prevent splashing and wet leaves that start the fungus growing.
Harvest fruit as soon as they are ripe. Clean up previous season’s plant debris and keep weeds that may harbor the fungus away from the crop zone.
If necessary, apply fungicides when the plants form their first fruit clusters and ensure complete coverage of the fruit. Copper based fungicides are considered safe to prevent anthracnose on tomato even if used up to the day prior to harvest and are registered for organic use if applied within guidelines.
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Anthracnose is a fungal disease of corn, cucumber, beans, peppers, squash and tomato. It can spread very quickly in warm (80 degrees F), wet weather, especially if air circulation is poor. Fortunately for California gardeners it doesn’t thrive in our hot dry summers.
This disease first appears as small, variously colored, circular spots (those on watermelon are angular) on the older leaves, though it eventually spreads to younger leaves, stems, pods and fruit. The spots enlarge and merge, getting darker until the leaves drop off and the plant is defoliated (or the stem is girdled) and dies. Sunken, round, water-soaked spots appear on fruit.
Anthracnose prevention is easier than cure. Remove diseased plants promptly to minimize its spread. Keep the plants off of the ground on stakes or cages to provide good air circulation. The spores overwinter on volunteers and crop debris, so clear up the beds in fall and rotate your crops. The spores are most often spread via water, when soil containing spores are splashed onto the plants by rain or irrigation. You can reduce this by mulching around the plants and by using drip irrigation. They may also be spread on the hands if the gardener, so don’t touch wet plants (especially not after removing infected plants). Some crop varieties are resistant to Anthracnose.
Anthracnose can also be carried on the surface of the seed, in which case treat them with hot water (127 degrees F for 25 minutes) or bleach solution (1 part bleach to 10 parts water for 30 minutes) to kill the spores.
Image: David B. Langston, University of Georgia, Bugwood.org
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What Is Anthracnose?
Anthracnose on a maize leaf. Source: IITA Image Library
The term “anthracnose” is used to describe a particular set of symptoms caused by an entire genus of fungi. The Colletotrichum genus is the source of most anthracnose-based symptoms.
Likely the best-known of these fungi is Colletotrichum gloeosporioides, a particularly problematic species that impacts hundreds of plant types. It causes damage to grasses and cereal crops, fruits and vegetables, legumes, perennial crops, and trees.
While C. gloeosporioides does a lot of the damage, anthracnose is not limited to that one fungal species… or even to that one genus.
Tomato anthracnose is caused by Colletotrichum coccodes, as an example. Sycamore anthracnose, however, is caused by Apiognomonia veneta, a completely different fungal genus.
There’s quite a few different causes, but thankfully the treatment for anthracnose will be similar despite different fungal causative agents.
I say “tomayto” and you say “tomahto” but however you say it, tomato time is here! Along with the luscious fruits that we gardeners await, we have the tomato disorders and diseases that the flesh is heir to—tomato flesh that is.
In my area, we have had enough rainfall this summer so no problems with blossom-end rot this year. This usually begins as a sunken spot on the blossom end of the fruit which turns black and leathery as it grows larger. Often you don’t notice it until you go to pick the tomato and find that the bottom half has rotted away—not a nice surprise! It is a physiological disorder rather than a disease caused when the plant has trouble extracting enough calcium from the soil because moisture levels are too high or too low. There can be plenty of calcium in the soil but the plants are incapable of utilizing it properly. Stressed plants divert the little calcium they have away from the fruit and send it to the shoots to keep them growing. Along with uneven moisture, excessive nitrogen and high soil acidity can contribute to blossom end rot. The best way to prevent it is to mulch your plants to keep the area around the roots consistently moist. See more about blossom-end rot solutions.
Early blight is the most common tomato disease.
It is a fungus that begins on the lower leaves as brown spots which enlarge into concentric rings like a bull’s eye. Eventually they get bigger and run together.
The lower leaves turn yellow and drop off, usually without affecting the fruit.
Sometimes dark patches will appear on the plant stems and on the stem end of the fruit. We have one ‘Early Goliath’ plant that shows signs of early blight but it soldiers on, has plenty of tomatoes forming (only one of those had stem end rot), and it keeps on blossoming. I keep plucking off the infected lower leaves and it continues to grow so for now it stays. I may regret that decision later.
One year we got it early in the season and watched helplessly as it turned all the plants and fruits to disgusting mush, practically overnight. You can track the spread of late blight across the country at the website usablight.org. Though most of the varieties we grow are heirlooms we also grow some hybrid tomatoes for their disease resistance. We are trying ‘Mountain Magic’ this year which is bred to resist both early and late blight.
Anthracnose And Fungal Diseases
Anthracnose damages just the fruit with its 1/4 to 1/2 inch spots. Septoria is another fungus causing small brown spots with black centers to appear on the older leaves. Eventually they turn yellow and fall off. To prevent all fungal diseases be scrupulous when cleaning up plant debris in the fall. All old leaves and fruit, especially those that were affected by disease should be removed from the garden and disposed of in the trash rather going to the compost pile.
Many tomato varieties are bred for disease resistance. Verticillium and fusarium are two wilt-causing diseases that have no cure. When shopping for tomato seed, look for the letters V and F after the variety name indicating resistance to those diseases.
Other letters are code for tolerance to other diseases: an A means the plant is resistant to alternaria, LB stands for late blight, EB early blight, N is for nematodes, T is tobacco mosaic virus, St is stemphylium leaf spot, Tswv is tomato spotted wilt virus, and Tylc is tomato yellow leaf curl virus.
With all that can befall a tomato plant from the time of germination to the picking of the first ripe tomato, you might think that it’s a miracle that we get any fruit at all! However, growing tomatoes is all about avoiding some common pitfalls that can trip you up along the way. Knowing what to expect and what to do about it will greatly improve your chances of a truly terrific crop of tomatoes.