Fate of PCNB
As you might know, the United States Environmental Protection Agency recently issued a 'stop sale' on all products containing active ingredient PCNB (pentochloronitrobenzene). Practitioners will no longer be able to purchase fungicide products containing PCNB, but existing stocks are still authorized for use until they are depleted. PCNB has been well recognized as the most cost effective and efficacious fungicide for controlling snow molds (Typhula blight and Microdochium patch), particularly on golf course fairways.
Are there any alternative chemicals to PCNB for the snow molds? Indeed, there is no single fungicide as effective as PCNB for snow mold management. Many of the fungicides listed below have some activity against snow molds. In most cases, however, a combination of these active ingredients must be tank-mixed for effective control. An unfortunate reality is that mixed treatments are more costly than PCNB. Furthermore, due to significant interactions between snow mold fungal species (or isolates within the species) and fungicides, unpredictable environmental conditions, and variable golf course budgets, it isn't easy to make a general recommendation of tank-mixes for snow mold management on all golf courses. Listed below are some cultural and chemical options to be considered.
The following fungicide classes most effective for snow mold management are
- Aromatic Hydrocarbon (contact): chloroneb
- Benzimidazole (penetrant): thiophanate-methyl
- Carboximide (penetrant): flutolanil
- Demethylation Inhibitor, DMI (penetrant): fenarimol, metconazole, myclobutanil, propiconazole, tebuconazole, triadimefon, triticonazole
- Dicarboximide (penetrant): iprodione
- Nitrile (contact): chlorothalonil
- Phenylpyrrole (contact): fludioxinil
- Strobilurin (penetrant): azoxystrobin, fluoxastrobin, pyraclostrobin, trifloxystrobin
Under high disease pressure, application of a single active ingredient cannot be expected to provide acceptable control. Effective control is often achieved with a two-way tank-mix under low disease pressure, but damage may approach 20-30% with a two-way tank mix under high disease pressure. If 20-30% damage is unacceptable, then a three-way tank-mix should be considered.
Suggested two-way tank-mixes for sites with low disease pressure:
- DMI + Phenylpyrrole
- Strobilurin + Dicarboximide
- DMI + Strobilurin
- Strobilurin + Nitrile
- DMI + Carboximide
- Nitrile + Dicarboximide
Suggested three-way tank-mixes for sites with high disease pressure:
- DMI + Dicarboximide + Nitrile
- DMI + Strobilurin + Nitrile
- DMI + Phenylpyrrole + Nitrile
- DMI + Strobilurin + Dicarboximide
Bear in mind that under high disease pressure 10-20% damage is somewhat common even with a three-way tank-mix. Luckily, spring recovery from damage levels from 30-40% or more is often achievable in a matter of weeks under New England growing conditions.
Disease Profiles and Cultural Options
Snow molds are patch diseases caused by various cold-loving fungal pathogens with damage appearing most often in early spring as snow melts. Typhula blight and Microdochium patch are the two most common snow mold culprits on turf areas such as home lawns, sports fields and golf courses. The following pathogen profiles provide some cultural practices for the management of snow molds:
Typhula Blight (Gray and Speckled Snow Molds)
Pathogens: Typhula incarnata, Typhula ishikariensis
Hosts: Most susceptible - bentgrasses, annual bluegrass, tall fescue. Moderately susceptible - perennial ryegrass, fine fescues. Least susceptible - Kentucky bluegrass, colonial bentgrass.
Optimum Conditions: Cold (30-40ºF), wet weather; prolonged snow cover; high nitrogen fertility.
Symptoms: Melting snow reveals circular gray or straw colored patches from a few inches to three feet or more in diameter. The grass in these areas is usually matted down and grayish-white mycelium is often visible at the edge of the patches. Sclerotia (resting structures) of the fungi can often be found in and among the diseased grass blades. Susceptible turfgrasses are usually severely thinned or may even be killed. T. incarnata has large, rust-colored sclerotia, while the sclerotia of T. ishikariensis are smaller, spherical and black (about the size of a pinhead).
Cultural Control: Plant less susceptible Kentucky bluegrass varieties for home lawns, sports fields and/ or colonial bentgrass for golf courses fairways and tees. Avoid heavy, (> 0.5 lb/1000ft2) late season applications of water-soluble nitrogen and continue to mow the grass until dormancy in the fall. Manage excessive thatch, reduce soil compaction, and topdress in the fall. Avoid compacting snow where damage has previously occurred. In the spring, rake and fertilize damaged areas to stimulate turfgrass re-growth from viable crowns since only leaves are killed in most cases. Remove snow and improve drainage in early spring so that snowmelt will dry rapidly in disease susceptible areas. Severely damaged turf may require reseeding. Preventative fall fungicide applications are critical on highly managed turf areas. Curative applications of systemic fungicides in late winter or early spring are not effective.
Microdochium Patch (Pink Snow Mold)
Pathogen: Microdochium nivale
Hosts: Most susceptible - annual bluegrass, bentgrasses. Moderately susceptible - perennial ryegrass. Least susceptible - Kentucky bluegrass, fine fescues.
Optimum Conditions: Cool-cold (30-60ºF), wet weather; high nitrogen fertility; alkaline soils; snow cover (not necessary).
Symptoms: Wet grass is covered with circular patches of tan to white grass ranging in size from a few inches to a few feet in diameter. The leaves of affected turfgrasses often mat together and pink mycelium is commonly visible at the edge of the patches. Unlike Typhula spp., M. nivale does not produce sclerotia. Mycelium and asexual spores (called conidia) of the fungus give the border of the patches a distinct pink to reddish brown color. In the absence of snow cover disease symptoms are different; in cool and wet weather, water-soaked patches of grass one to a few inches in diameter are common. These patches are grayish to white in the center with reddish-brown borders. At higher temperatures (50-65ºF), the fungus is capable of causing a yellow blighting of higher cut grasses during wet periods.
Cultural Control: Plant less susceptible turfgrass species such as Kentucky bluegrass or fine-leaved fescues. Avoid late season applications of readily available nitrogen and mow the grass until dormancy in the fall. The disease is more severe under alkaline conditions, so maintaining a pH of 7.0 or less in the soil profile will reduce disease damage. In the spring, rake and fertilize damaged areas to stimulate turfgrass regrowth. Severely damaged turf may require reseeding.
Written by: Dr. Geunhwa Jung & Katie Campbell-Nelson