When to Stop Spraying Fungicide

For many vegetable growers, the season is in full swing. All that hard work in season preparation, planting and maintenance is paying off with harvest. One of the on-going season maintenance issues is applying fungicides. In other articles, I have described how and when to spray. In this article, I want to address when to stop. To limit the scope of this article, I will concentrate on tomato, cantaloupe and watermelon crops. These are crops where the fruit is consumed, not the foliage.

For most vegetable crops, there is no need to apply a fungicide shortly before the final harvest. Foliage needs to be protected to preserve fruit quality. A plant with reduced foliage will produce a smaller fruit and/or fruit that have fewer sugars and other desirable compounds. I don’t know how much foliage needs to be reduced to affect fruit size or quality. However, I do know that for many foliar diseases, symptoms will not be obvious for a week to 10 days. It will take even longer for the foliar disease to significantly reduce foliage. Therefore, for many diseases, it doesn’t make much sense to spend good money for a fungicide on a crop that is 2 to 3 weeks before the final harvest.

Examples of diseases that affect foliage, but not fruit directly include:  powdery mildew of cantaloupe or tomato, early blight of tomato, Septoria leaf blight of tomato, gummy stem blight of watermelon or cantaloupe, Alternaria leaf blight of cantaloupe. With some rare exceptions, these diseases reduce yield or fruit quality by affecting foliage, not by attacking fruit directly.

Diseases that affect fruit directly may need fungicide applications closer to harvest. A disease that can cause a lesion directly on a fruit can ruin the marketability of the fruit or even cause the fruit to begin to rot in transit. However, most fungicides will remain active in or on the plant for 6 to 7 days even during the most conducive weather. Therefore, an application of a fungicide to protect fruit from direct infection from disease is probably not necessary within 7 days of the final harvest.

Examples of diseases that may affect fruit directly include:

  • Anthracnose of watermelon-this disease can cause loss of foliage, but also lesions on the fruit. An infection on the day before harvest could, theoretically, cause a lesion in transit.  During weather that is conducive to disease, it makes sense to keep a fungicide on the plant surfaces during the last several days before harvest. Growers that are using the MELCAST system will be able better judge when the weather is conducive for anthracnose.
  • Phytophthora blight-this disease affects foliage as well as fruit. As with anthracnose above, a lesion that develops before harvest could start to rot the fruit in transit. Specialized fungicides applied 7 to 10 days before final harvest should protect the fruit.
  • Bacterial spot or speck of tomato-lesions of these diseases that occur on the fruit can ruin marketability. Applications of a copper product should help to protect the fruit during the last week or so. Warm, wet weather shortens the disease cycle and increases the likelihood of infection.
  • Bacterial spot of pumpkin-this disease can cause pimple-like lesions that may ruin marketability. However, the disease affects fruit during the first 14 days or so after pollination.  After this period, infection is much less likely due to changes in fruit maturity. Therefore, copper applications during the last weeks before harvest make little sense.
  • Blossom-end rot-trick term! BER is not a disease at all. Instead, BER is a calcium deficiency often brought about by uneven moisture in the soil around the root. No amount of fungicide at any timing will help to slow BER. The point is to know what the fruit problem is so that you will know what to do-and what not to do-to reduce the problem.

Pre-harvest Interval (PHI) – when applying fungicides close to the final harvest-or any harvest-keep in mind the PHI. Often growers will need to change what fungicide is used when vegetables reach harvest stage. For example, cantaloupe growers may decide to use a fungicide with the active ingredient mancozeb PHI 5 days early in the season (examples include, Dithane®, Manzate®, Penncozeb®, Roper®). As harvest grows near, however, a fungicide with the active ingredient chlorothalonil might be used since it has a 0 day PHI (examples of products with chlorothalonil include Bravo®, Equus®, Initiate®).   The PHI for each crop can be found in the fungicide label with the appropriate crop grouping.

Finally, one should be realistic about applying fungicide to a field of vegetables that is severely diseased. The following article discusses the pros and cons of such late season applications  https://vegcropshotline.org/article/to-spray-or-not-to-spray/  Remember, no fungicide will turn brown plant tissue back to green.

I am glad to discuss any special circumstances about deciding when to make the final fungicide application.

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