Cercospora spp., Cercospora fungi Photo: Yonghao Li, The Connecticut Agricultural Experiment Station, Bugwood.org
These organisms are classed by their structures, and are quite numerous in species (Species Fungorum lists 133,685 species of Cercospora for example). Cercospora, Alternaria, Anthracnose, Ascochyta, Corynespora, Cylindrocladium, Cylindrosporium, Didymella, Entyloma, Fabraea, Marssonina, Phyllosticta, Pleospora, Ramularia, Septoria, are some of the most common leaf spotting genera of fungi, but there are others. Some of theses disease organisms will attack a wide range of host plants others are specific to one or a few species, or even varieties.
The leaf spot damage will vary from minor to severe depending on the interaction of host plant, fungus, and environment. Leaf spots can vary in size, shape, and color depending on the host-fungus combination. Some leaf spotting fungi produce fruiting bodies, that are typically dark colored, other produce distinctive masses of spores. A hand lens or microscope will help spot the clues. Some produce no conspicuous signs. As the infection expands the spots can merge forming larger dead areas which are termed blights.
In severe cases all of the foliage can become blighted. Frequently the spots are actually concentric circles of damage, showing different colors from the center of the damage to the outer edge. This is a good diagnostic tool for identification of the causative organism. Almost all leaf spot diseases thrive in warm, moist weather. Spores need leaf surfaces to remain wet for a few hours to many hours for infection to occur. This is also dependent on host plant and temperature as well.
Extended periods of warm rainy weather can bring on severe leaf spot fungal outbreaks. Many leaf-spotting fungi are very capable of surviving unfavorable periods such as winter temperatures, or dry periods. Some are also capable of infecting weed species in between host plant availability. Management of the diseases caused by leaf-spotting fungi will vary by crop size and value. If you are growing plants for pleasure, these organisms may not need immediate control, but if the plants are part of a commercial operation then prompt application of control methods is required before the problem spreads causing further losses.
Good cultural controls
- Reducing or discontinuing overhead irrigation
- Proper plant-to-plant spacing to speed up canopy drying
- Watering earlier in the day to promote leaf drying between irrigations
- Careful examination of plants coming into an operation or landscape setting to detect any fungal infection is always a good policy
- Removal of infected plant parts by pruning and take the infected material away from the site, and disposing of it in a closed plastic bag in the trash can or dumpster. Removal of infected parts is one of the principals of disease control. When you deny the leaf spotting fungi their hiding places in dead and damaged tissue you can reduce the risk of subsequent out breaks of the disease.
A wide arrange of fungicides are available for the control of leaf spotting fungi. A good number of the products are effective against a number of fungal species. Many of these products are most effective when used as a preventative measure, before symptoms actually appear. Spray frequency can be reduced in periods of dry weather.
In the application of fungicides in a rotational program, use a product with a different fungicide resistance activity committee code (FRAC). By doing this rotation resistance to the product will be less likely, than if you are applying similarly coded products. Remember it is the law to apply the product in accordance with the label instructions. The label is the law!