Black mold (alternaria) on tomato fruit

There are many potential causes of fruit rot in tomato. In processing crops, we often see them when crop maturity is getting ahead of harvest.


Anthracnose on tomatoThe most important fact to know about anthracnose fruit rot of tomato is that while symptoms appear only on ripe fruit, infections can be initiated on green fruit (you can’t see those infections). Fungicide programs must begin early enough to prevent the initial infection of green fruit. You can’t spray away an infection that’s already happened.

Black mold (alternaria)

Black mold (alternaria) on tomato fruitOverripe tomato fruit may develop black mold caused by Alternaria alternata. Symptoms can range from small, dark blotches to large sunken areas Lesions may develop soft, black fungal growth in warm, humid weather. Black fungal growth may also develop on existing wounds or lesions.

Bacterial soft rot

Bacterial soft rot of tomato
Bacterial soft rot of tomato

Bacterial soft rot results in the familiar “bag of juice” symptom, where the tomato flesh liquifies quickly, while the skin remains relatively intact (for a time). As a bacterial disease, it is not directly affected by fungicides. However, fungal disease lesions may provide a point of entry for the soft rot pathogens. Insect damage, growth cracks, blossom-end rot, or other physical damage can also provide wounds where soft-rot infection can begin.

Managing fungal fruit rots

Fruit rot problems are exacerbated by:

  • Abnormally warm temperatures in late summer and fall
  • Premature defoliation (foliar disease or too high a rate of Ethrel)
  • Crop maturing before scheduled harvest
  • Harvest delays due to heavy rainfalls

According to research by Cheryl Trueman at Ridgetown Campus, regular applications of fungicides should begin at early fruit set. Timely fungicide applications beginning at fruit set are the most important fungicide strategy. The infections of green fruit need to be prevented to avoid problems when the crop matures.

Continuing fungicide applications close to harvest may reduce new infections of anthracnose (and other fungal fruit rots, such as black mold) compared to ending the fungicide program 28 days before harvest, although this did not result in an increase in marketable red yield in the research trials. If the crop is maturing ahead of the harvest schedule, there may be some benefit to continuing chlorothalonil applications. When high temperatures persist during tomato harvest, development of fruit rots will accelerate. Secondary infections of damaged tissue are a concern as well. Warm tomatoes in the load will break down more quickly, too.

If Septoria leaf spot (or early blight?) is present and harvest is delayed:

  • Fungicide applications up to 7 days before harvest may help reduce total rots
  • Including Quadris may further reduce total rots

In most cases, control of anthracnose was similar with Bravo 500 or with Quadris alternated with Bravo 500.

If there is significant risk of late blight, continue late blight fungicide applications almost to harvest to protect the crop. In this case, Quadris would not be included in the program as it is weaker on late blight.

Managing bacterial fruit rots

These can be very difficult to manage, especially when conditions are warm and wet. Control other fruit diseases — those lesions can be points of entry for soft rot bacteria. Manage fruit-feeding insects that provide entry wounds and a means of transport for the bacteria. Once the disease is identified in a field, avoid overhead irrigation. Heavy plant canopies that don’t dry out would be expected to exacerbate the problem.

Related posts:

Fungicide efficacy summary tables for management of diseases in field tomatoes

Managing field tomato fungal disease

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