Control Center: Fall Infections

Disease control in the fall often gets put on the back burner, especially if it’s been a tough summer. When faced with a bunch of rough and tumble problems, scouting for potential upcoming fungal diseases is not high on the to-do list.

However, as we all know, the problems we put off today are usually harder to deal with tomorrow.

Maladies such as rust, powdery mildew and leaf spot may not be a factor in 2014. Even so, it’s prudent to routinely scout for their development, keeping in mind that weather and other conditions may accelerate or delay their onset.

Rusted turf

Easy to see, especially if you happen to be wearing white canvas shoes, heavily rusted turf appears yellow or orange when seen from a distance. Clouds of orange rust spores are commonly observed discoloring pant legs and grass catcher bags as well as shoes. Close-up examination of rusted leaf blades reveals the presence of orange to brick-red pustules. Spores within the pustules rub off easily when touched. Each one produces a vast number of spores, each of which is capable of infecting a grass blade.

Caused primarily by Puccinia spp., rust diseases occur on most commonly grown turf species. Like the mildews and smuts, rust fungi are highly specialized as to host preference. For example, two of the more common species are P. graminis, causing stem rust on Kentucky bluegrass, and P. coronata, which causes stem rust on ryegrass and tall fescue. The severity of rust outbreaks varies from year to year.

Rust usually occurs from mid to late summer until mid-fall following hot, dry periods when grass growth has slowed. It becomes severe when influences such as lack of water, low fertility or soil compaction reduce turf vigor and growth. Warm days and moderate nighttime temperatures along with long dew periods or night watering create the optimal environmental conditions for rust outbreaks.

For infection to occur, the turf must stay continually wet for six to eight hours. Rust can survive on infected plants during mild winters in the North and in regions known for mild conditions. However, in the central and northern plains states, most of the rust overwinters in the south, with the spores carried north by winds during the growing season. The variable survival of the fungus is one reason for the sporadic nature of the disease and late-season buildups of rust.

Under ideal conditions, turf can become heavily rusted about 40 days after the initial infection. Moderately to heavily rusted turf stands become weak and thin, making them susceptible to winter injury and broadleaf and grassy weed invasion, as well as damage from various other environmental stressors.

Rust management begins with the use of improved rust-resistant cultivars. Maintaining turf in a vigorous but not lush condition through proper fertilization, early-morning watering and aeration to alleviate compaction are recommended to prevent injury from rust.

An application of nitrogen fertilizer can help a turf stand recover from an outbreak of rust. In addition, regular mowing also reduces the potential for severe rusting in late summer and fall.

Fungicide applications should be considered on turf with a history of rust infection. Generally, the initial application should be made in midsummer, followed by one to two additional treatments at three-week intervals, but the initial timing will depend on geography and rust overwintering each year. Several products are available to choose from including azoxystrobin, chlorothalonil, mancozeb, propiconazole and triadimefon.

Powdery mildew

Another easy-to-identify fall disease is powdery mildew. It occurs on several cool-season grasses including Kentucky bluegrass, turf-type tall fescues and fine fescues, primarily in shady locations. Powdery mildew is caused by the fungus Blumeria graminis. Although it has the reputation of being a late summer or fall disease, it can occur in the spring, summer and fall if conditions allow for its growth and development.

The mildew fungus attacks the surface of the grass blades where it produces a white to light-gray powdery growth. Infected leaves become yellowed and later turn tan or brown. Turf in heavy shade looks like it has been dusted with flour or lime. Turf stands that are comprised of susceptible cultivars or newly seeded turf can become thinned by severe infection.

This highly specialized fungus can attack only certain cultivars of a few turf species. Experimentation with various cultivars may be helpful in determining appropriate genetic resistance. Powdery mildew develops in areas of dense shade where air movement is poor. Moderate temperatures, high humidity and cloudy conditions favor outbreaks of mildew.

Because turf areas under large shade trees and along the north and east sides of buildings, such as clubhouses, refreshment stands and bathrooms, are particularly vulnerable to mildew infections, consideration of tree removal is a reasonable step. Likewise, under the guidance of the right plant, right place principle, replacement of turfgrass with shade-adapted ground covers may be a prudent option, especially if the area is not routinely in golf play. If turfgrass is required, renovation of affected areas to incorporate shade-tolerant tall fescue cultivars is helpful in terms of powdery mildew control.

Fungicides can be used to treat high-value turf with a persistent powdery mildew problem. One or two applications in spring, summer and fall provides effective protection against turf loss from mildew. Effective products include myclobutanil, propiconazole and triadimefon.

Leaf spot and melting out

Leaf spot is commonly considered a spring and fall disease. Caused by Bipolaris andDrechslera spp., these fungal diseases are within the Helminthosporium leaf, crown and root disease complex. Bipolaris is active during cool periods and attacks many turf species including Kentucky bluegrass, tall fescue, creeping bentgrass and perennial ryegrass. Melting out, caused by Drechslera poae, affects most turf species as well, commonly as a warm-weather pathogen.

Both diseases are favored by dry periods that alternate with prolonged cloudy, wet weather. Under these conditions, both pathogens produce large numbers of spores on plant debris. These spores are spread to new growth by wind, mowers and other turf equipment. The spores germinate soon after contacting water droplets on the leaf. The fungus can enter the leaf within hours. Symptoms appear soon after, and a new crop of spores can be produced within seven to 10 days.

A variety of leaf spot symptoms accompany the stages of disease development. Early symptoms are small, dark purple to black spots on the leaf blades. Later symptoms are round to oval spots with buff-colored centers surrounded by dark brown to purple margins. Melting out starts out as black to purple spots on the leaf blades and sheaths. Infected leaf sheaths result in bases having a uniform dark chocolate brown color and causes leaves to yellow and then drop from the plant. Looking at an infected stand from a distance, affected turf appears yellow and thin.

Symptoms on bentgrasses differ from those on other turf species. Infection of low-cut bentgrass produces a smoky-blue cast in the turf that progresses to a yellowing and finally a complete blighting of the leaves and thinning of the stand. Leaves within the affected areas are water-soaked and matted. On bentgrass fairways, initial symptoms are yellow flecks on the leaves that develop into small oval lesions and then into irregular water-soaked blotches.

The most effective control program for leaf spot and melting out combines the use of improved cultivars, good management practices and fungicidal spray applications. Fertilization regimes that encourage steady, vigorous growth; thatch management; and watering in the morning to reduce the number of hours that the turf blades contain free moisture for disease development are appropriate measures.

Azoxystrobin, fludioxonil, fluoxastrobin, mancozeb, penthiopyrad, pyraclostrobin and vinclozolin are among the most effective products for these diseases. When necessary, application of a fungicide product beginning with the onset of cool conditions in fall, followed by two to three additional applications spaced three weeks apart is recommended.