Vegetable growers are used to scouting for pests such as spider mites and aphids. Growers have come to recognize the yellow leaves caused by spider mites and the curled leaves caused by aphids. Growers understand that even after spider mites and aphids are dead, the symptoms of the damage may remain on the affected leaves.
Whereas, a leaf distorted by aphids remains distorted even after the aphids are dead, a leaf with symptoms of a foliar disease such as anthracnose of watermelon or early blight of tomato typically contains viable spores even after repeated fungicide applications. So, what is the purpose of fungicide applications?
Contact fungicide applications with active ingredients such as chlorothalonil (e.g., Bravo®, Echo®, Equus®, Initiate®) or mancozeb (e.g, Dithane®, Manzate®, Roper®, Penncozeb®) are used to coat the surface of the leaf so that when spores land on a leaf, they are neutralized. While it is true that a contact fungicide may kill spores on the surface of a lesion, the fungus in the interior of the lesion typically remains alive. Products with copper are more effective against bacterial diseases than fungal diseases.
Systemic fungicides will move within a leaf. Some products may move from one side of a leaf to another. Other products may move a half inch or so in the direction of the growing tip of the plant. Some systemic fungicides will move into a small lesion before it starts to produce spores or even shortly after the lesion begins to produce spores. Large, easily visible lesions, however, are not likely to have the pathogenic fungus completely eliminated from the lesion.
When one looks at a lesion, the observed brown, necrotic lesion is where the fungal pathogen has begun to kill the tissue. If one had magic glasses, it would be possible to see that the fungus most likely extends beyond the lesion to parts of the leaf that aren’t brown yet. It is possible that an effective systemic fungicide may begin to act on the fungus from the edge of a lesion. In this way, the size of the effective lesion may be reduced. However, most visible lesions will still retain viable pathogen spores.
How do I know this? Because when I take a leaf with a lesion of, say, anthracnose of watermelon into the lab, I can often see spores being produced on the surface of the lesion. Sometimes, it is necessary to incubate the lesion in a humid plastic bag overnight before the spores become visible. In addition, I can easily isolate the spores from the lesion onto a Petri dish. Often these leaves have had repeated fungicide applications.
The purpose of fungicide applications is to slow down the spread of the disease primarily by protecting healthy green tissue. We know that without fungicides, the disease may, under conducive conditions, cause severe economic loss to the grower. However, fungicides are unlikely to cause the fungus in a lesion to be completely killed in the same way that an insecticide will kill an insect.
Much the same can be said of bacterial lesions. The bacteria in a lesion of bacterial spot of tomato, for example, are not likely to be completely killed by copper applications. The purpose of copper applications is to slow down the spread of bacterial spot. In fact, most copper applications are on the surface of the plant only.
The purpose of this article is not to discourage growers. However, it is helpful to know how fungicides help to slow down foliar diseases, even if it is not possible to kill all the pathogenic fungi in a field. Cultural management techniques like crop rotation and regular preventative fungicide applications will help to reduce disease pressure. In the case of cantaloupe and watermelon, the weather-based disease-forecasting program MELCAST will help growers to apply fungicides when they are most needed.