Net Blotch Can Be Averted


Net blotch causes brown lesions on the leaves of tall fescue, but can usually be controlled through cultural practices.

Primarily a disease of home lawns and parks, net blotch is a minor infection that can be easily avoided by growing healthy grass and using an awareness of the fungus to watch for symptoms. Since its primary host is tall fescues, it is a disease of cool-season and transition regions. At its worst it can be treated with fungicides at the same time as other fungal diseases that occur in the same location.

Lane Tredway sees net blotch occasionally in the transition zone, where he is an associate professor and extension specialist in turfgrass diseases at North Carolina State University in Raleigh. He says the fungus is a minor problem, confined mostly to home lawns and golf course roughs, as well as parks and other municipal plantings in the state.

A plant pathologist whose Ph.D. focus was on gray leaf spot, Tredway is familiar with fungal diseases of turfgrass around the world, and he puts net blotch low on the priority list for treatment. It is part of a complex called Helminthosporium diseases caused by cigar-shaped fungal spores, but with a reclassification in taxonomy the responsible fungus is now named Drechslera dictyoides.

“Practically every grass species we grow is susceptible to one of these Helminthosporium diseases,” Tredway says, but they generally have a low impact on turfgrass health. Such is the case with net blotch, which causes small brown spots on grass leaves that can coalesce into a net-like pattern of damage. However, it is rare that the disease becomes so prolific. “Frankly, I’ve never seen that net-like pattern that people talk about,” he says.

Tall fescue is a utility grass in the area, and it is often used in shaded areas where it is more susceptible to net blotch and other fungal diseases. Net blotch occurs early in the spring and into early summer, as well as in the fall, when temperatures are in the 60 to 65-degree range. Overwintering in the thatch of previously infected tissue, it develops largely at night when moisture is on the leaves and temperatures are cool. It can be enhanced by periods of cloudy or rainy daytime weather.

Even during these periods, net blotch is rarely seen in stands large enough to concern landscape or parks managers, Tredway says, but it can occur alongside brown patch, a disease that is serious and often treated. On its own, net blotch will naturally die back as the heat of summer comes on, and grass will recover during that period without significant controls if good turf management is used to promote thriving grass.

Cultural controls may be all that is needed to curb this foliar disease. Tredway says the first object is to prevent it altogether by using resistant cultivars of tall fescue. Turf managers who are planting new plots or renovating old ones should consult the test results from the National Turfgrass Evaluation Program or data from universities near the planting site to find cultivars resistant to leaf spot diseases.

“A key point is to select a good variety of tall fescue that will do well in your area,” he says, as the health of the grass is a crucial component in the prevention of net blotch and other fungal diseases. Tall fescue blends are also more desirable than plantings of single cultivars.

Ultimately, good turfgrass management will curb net blotch in almost all situations. Tredway points out that overfertilizing is a practice to be avoided, with the general rule that nitrogen not be applied in amounts over 1 pound per 1,000 square feet at one time. Irrigation management is also crucial. Irrigate as infrequently as possible during the May to June period, using deep waterings, in order to reduce plant susceptibility. Keep in mind that soil moisture is not the issue; leaf wetness is what promotes net blotch.

Tall fescue isn’t a big thatch producer, but managers should be aware that thatch is where the organism survives in the off-season. Thus, excessive thatch should be removed. Mowing height of the fescue should be at about 3 inches, and mower blades should be kept sharp to avoid leaving ragged leaves that could be more vulnerable to the fungus.

Tredway says he almost never sees anyone using fungicides to treat net blotch, but that doesn’t mean it can’t be kept under tighter control by treating brown patch. “A lot of the fungicides we use for brown patch could be used for net blotch as well,” he says, and the two diseases can overlap in May and June. That is when turfgrass managers use chemicals as a preventative measure against brown patch.

Based on research test data and manufacturers’ information, Tredway lists some of the top-performing fungicides. Those include iprodione (26GT, IPro, Iprodione Pro, Raven), as well as iprodione mixed with thiophanate-methyl (26/36, Dovetail, Fluid Fungicide). They also include vinclozolin (Curalan, Touche), mancozeb (Fore, 4 Flowable Mancozeb, Dithane and others) and mixes of mancozeb with myclobutanil (Manhandle). The mancozeb mix with copper hydroxide (Junction) has also proven to be effective. Azoxystrobin (Heritage) and azoxystrobin blended with propiconazole (Headway) also make the list.

Chlorothalonil (Daconil, Chlorostar and others), as well as its blends with thiophanate-methyl (Spectro, ConSyst and others), propiconazole (Concert) and propiconazole with fludioxonil (Instrata) are effective at a somewhat lower level. Pyraclostrobin (Insignia) is also in this group. For a complete North Carolina State list of appropriate fungicides and their efficacy levels go to eases/Net_Blotch.aspx.

Pay attention to labels, since most of these chemicals are not registered for home lawns, and some are for golf course use only. Tredway notes that the QoI fungicides are the chemical class that is used most frequently for brown patch control in tall fescue, and these products are generally also effective against net blotch. The QoI fungicides are azoxystrobin, fluoxastrobin (Disarm), pyraclostrobin and trifloxystrobin (Compass). He highlights these as the ones to use if needed to also help control brown patch.

Again, Tredway emphasizes that rarely is net blotch a disease that will kill significant patches of turfgrass, and it is usually treated secondarily when brown patch is treated. If only spraying as a control for net blotch, he surmises that a single application could suffice. Or, the turf manager can wait for the return of warm weather and place his emphasis on growing healthy tall fescue that can withstand this fungus.

Don Dale resides in Altadena, Calif., and is a frequent contributor to Turf. He has covered the green industry for more than 10 years.