Foliar and Basal Rot Anthracnose

A review of the literature reveals that superintendents are not alone in their struggle with anthracnose, a disease that is not unique to turfgrass. In plants, any leaf disease which is caused by a fungus that produces acervuli (saucer-shaped, asexual fruiting bodies that produce conidia beneath the epidermis of a plant and rupture at maturity) is referred to as anthracnose. Historically, anthracnose in turf was thought of as a disease of minor importance, but foliar and basal forms of anthracnose are now considered major problems. Generally considered a weak pathogen, anthracnose occurs on a wide range of both cool season and warm season grasses throughout the world. In North America, its primary hosts are annual bluegrass (Poa annua L.) and bentgrasses (Agrostis spp.).

Disease Cycle And Symptoms
Anthracnose diseases in turfgrasses are caused by the pathogen Colletotrichum cereale (formerly called Colletotrichum graminicola). The expression of the foliar and basal rot forms of anthracnose are very distinct and form under different conditions.

The symptoms of anthracnose foliar blight, leaf yellowing and/or a reddish discoloration of the turf followed by a decrease in shoot density, are commonly seen during periods of high temperature stress. During warm, humid periods this fungus is a prolific spore producer. Conidia produced by very distinctive acervuli (the acervuli contain sterile black hairs known as setae that can be seen with a 10X hand lens) can initiate secondary infection cycles that can greatly increase the severity of the disease if left unchecked.

Anthracnose basal rot symptoms may appear at almost any time of year, but most outbreaks take place in early spring through mid summer. Plants turn yellow or orange (depending on the time of year) in irregular patches. The patches range from ½ to 6 inches in diameter in annual bluegrass or up to 18 inches in bentgrass. An inspection of infected plants often reveals a water-soaked or black rot of the crown tissue. Basal rot infection inevitably leads to plant death. Again, prolific spore production by the pathogen spreads the disease. Large areas of turf can be affected and the symptoms can at times be confused with other diseases.

The primary conditions necessary for infection by C. cereale are those that stress the plant. Low fertility (especially N), drought stress, temperature extremes, soil compaction, injury, mowing height, mowing frequency, growth regulators and herbicides have all been implicated as contributors to outbreaks of anthracnose. In addition to a stressed host, this pathogen also requires a moisture film for infection. Thus, outbreaks tend to be associated with conditions that increase moisture in the leaf canopy like overcast skies and high relative humidity.

Cultural And Chemical Control
When environmental conditions are not conducive to infection, C. cereale survives as saprophytic mycelium in infected plant debris. C. cereale in able to infect new turf plants when they experience a decline in vigor due to a variety of climatic, physiological, or physical stresses. Understanding this is key to preventing and managing this disease. Control measures are especially necessary during hot, humid weather in turfgrass stands comprised of a large percentage of Poa annua. Cultural measures including proper fertilization (particularly adequate N), reduction of soil compaction and excessive traffic, dethatching, increasing height of cut, improving air circulation, use of growth regulators and herbicides and prevention of drought stress are important factors in reducing disease severity. Chemical control measures were once believed to be of little or no benefit for this disease. However, it is now believed that fungicide programs can be effective within certain parameters:

1) Make preventative applications about 2-3 weeks before symptom onset on sites with a history of anthracnose. This disease is much more difficult to treat curatively once it has become established.

2) Rotate and tank mix different chemistries. Benzimidazole (including 3336 and 3336 PLUS), antibiotics (Endorse), strobilurin, nitrile (Spectro) and demethylation inhibitor classes have all been reported to consistently control anthracnose even when used alone. Phosphonates (Alude), dicarboximide (26/36 Fungicide) and phenylpyrrole classes all provide some control of anthracnose and make good tank mix partners.