by Dr. Tom Hsiang
GREY SNOW MOLD
Introduction: As we move away from the heavy metal protectant fungicides (e.g. Calo-clor, PMAS, Scotts Broad Spectrum Fungicide), turf managers will need to properly target their fungicide applications for particular snow mould diseases. The newer fungicides tend to be much more specific, and thus may not give broad spectrum control against all snow mould diseases. Grey snow mould and pink snow mould are two of the major winter diseases. The fungi which cause these two diseases belong to very different taxonomic groups, and thus may not be properly controlled by the same fungicides. In this issue, we'll discuss grey snow mould.
Other common names:
Typhula Blight, Snow Scald and Winter Scald
Typhula incarnata and Typhula ishikariensis
Host Plants: All cool season turfgrasses, particularly creeping bentgrass, annual bluegrass, and perennial ryegrass.
Season of Occurrence: Winter, early spring
Conditions Favouring Disease:
- -1°C to 4°C
- Extended periods of snow cover, usually greater than 3 months, although leaves, straw mulch, and greens covers may also create favourable conditions.
- Snowfall on unfrozen turf that has not been hardened by frost.
- High moisture, especially under melting snow or under snow on unfrozen ground. Deep snow cover prevents soil from freezing and the snow usually persists longer so that disease stays active longer.
- High nitrogen, especially in fall prior to dormancy, provides succulent tissues for the fungus.
- On long cut turf, there is general blighting rather than distinct patches.
- On close-cropped turf, patches appear in spring after snowmelt which have a scalded or bleached appearance ranging in size from 10 to 20 cm, but often overlapping to form large irregular patches.
- Greyish-white mycelium can frequently be seen on the outer margin of the patch, up to several days after snow-melt, often with sclerotia embedded in the mycelium.
- Grey snow mould damage may be found in combination with pink snow mould patches. The pink snow mould fungus does not produce sclerotia, however, in the absence of sclerotia, the disease may still be grey snow mould. It is only the presence of sclerotia that confirms the disease as grey snow mould.
- Sclerotia are over-summering structures that can resemble mouse droppings. They can be seen only after snowmelt, as large as 1/2 cm and visible to the naked eye. These are pinkish-brown to a dark-red-brown (from T. incarnata). There may also be much smaller pin-head size sclerotia that are brown to black (from T ishikariensis). Later in spring, the sclerotia dry up and are no longer visible. They are dormant throughout the summer.
- Disease tends to be present in the same areas annually if conditions remain the same.
- Minimize thatch since this is the habitat for over-summering sclerotia.
- Prevent succulent growth into late fall, by mowing until leaf growth stops, and not applying nitrogen any later than 6 weeks before dormancy. Dormant applications, particularly of slow release nitrogen fertilizers are not thought to favour grey snow mould.
- Prevent snow compaction due to snowmobiles, skis, etc.
- Use snow fencing, windbreak plantings or other methods to prevent formation of large snow drifts and to minimize snow accumulation.
- Physically remove snow or enhance snow melt with dark substances (e.g. fertilizer, topdressing) to minimize snow accumulation when the snow cover duration approaches 3 months, or if the snow is so deep that the melt occurs slowly over several weeks thus providing moisture for fungal diseases to occur.
- As damage becomes apparent after snowmelt, rake matted areas to encourage drying. Promote new growth bylightly fertilizing damaged turf. Seeding may be required if crowns are killed. You can check this by taking out a plug of turf and placing it under warm conditions to see if green emerges: if >50% of the plug is green after 10 days, then turf should recover; otherwise, on putting greens, you should reseed or resod since annual bluegrass will probably invade.
Several fungicides are registered and recommended for the control of grey snow mould in Canada. Most of these will also work against pink snow mould, but check local registrations and recommendation. Unlike pink snow mould, applications of fungicides are not necessary in through the fall to control grey snow mould because it is active only under snow cover.
- The chemical should be applied in the late fall just prior to snowfall. Note that the rates of application may be several times higher for snow mould control than for the summer foliar diseases.
- If there is rainfall after application, protectant fungicides (particularly newer ones) may have to be re-applied. Similarly, if there is a period of snow-melt in mid-winter, leaving grass bare and exposed, protectant fungicides may have to be re-applied.
Resistant Turfgrasses: The following are considered less susceptible to grey snow mould: Kentucky bluegrass, red fescue, and tall fescue, although grey snow mould disease can be found on these species.
Take-all patch is a severe disease of bentgrass turf was first reported on turf in Holland in the 1930's. In North America, it was found on bentgrass in 1960 in the Pacific Northwest. In the Great Lakes area, the first confirmed reports on bentgrass were made in the mid-1970's. In Canada, the first reports were from south-western Ontario in the mid-1980's. Since that time, positive identification of take-all patch has been made in southern British Columbia, southern Quebec and other parts of southern Ontario. Newly built bentgrass putting greens seem to be most susceptible to the disease, and with increasing greens construction, many more cases of take-all patch have been reported. The fungus causing take-all patch also causes a severe disease of cereals, and can also be found in cereal growing areas.
Other Common Names: Ophiobolus patch, Gaeumannomyces patch.
Pathogen: Gaeumannomyces graminis
Host Plant: Creeping bentgrass
Season of Occurrence: Spring and fall, with symptoms enhanced by summer stress.
Conditions Favouring Disease:
- Cool moist soils.
- Drought stress in summer.
- Poorly drained, irrigated turfs.
- Soil pH must be >6.5 for disease to occur. Sand with high carbonate content will have higher pH, and liming will also raise pH.
- Most common 1-3 years after turf establishment, but after that sometimes it will decline.
- New infection centres are often seen on greens in late spring. They are initially depressed circular patches, a few cm across, resembling Fusarium patch. These patches may eventually expand up to 1 m in diameter over several years.
- Symptoms of existing patches which overwintered may first be observed in the spring, but commonly do not fully develop until summer, and are enhanced by water deficits.
- In the summer, patches may look similar to brown patch but without a smoke ring.
- Resistant grasses or weeds can grow in the centre of patches giving a frog-eye appearance.
- The fungus causes root disease, and may act by plugging up the roots and causing wilt and death.
- On dead leaves of creeping bentgrass, there is tip dieback only, with no lesions, mycelium nor spore- producing structures.
- Infected roots will be dark brown, whereas healthy roots are ivory white. Look for dark roots in top 3 cm of soil.
- Dark threads called runner hyphae can be found on roots. Look for these with a 10X hand lens.
- In advanced stages, turf is easily pulled from the soil due to the root rotting.
- Patches can persist through winter or can reappear annually in the same spot, enlarging by 15 cm every year.
- Small black fruiting structures which have a pear shape can form on plant crowns during autumn. The neck of the black body protrudes through the dead tissue and can be seen with a 10X hand lens.
- The fungus survives winter and adverse conditions as mycelium in dead tissue in the soil and thatch.
- Activity starts in cool wet weather, but symptoms usually are not seen until warmer drier weather.
- Initial infections occur on roots and crowns, and the fungus spreads along root surfaces to adjacent plants.
- In fall, cool moist conditions allow fungus to grow and infect more turf.
- Control of take-all patch is much more difficult that control of foliar disease, because symptoms are seen only after extensive damage to the roots has occurred.
- Promote good rooting, such as by delaying spring nitrogen fertilization until a month after first growth.
- Control thatch to promote plant vigour.
- Adequate amounts of the micronutrients Mn, Mg and Zn and the macronutrient K may decrease take-all patch.
- Depending on the soil type and soil pH, ammonium fertilizers may decrease pH and inhibit disease.
- Overseed or resod patches with resistant turfgrasses.
No chemicals are registered on turf for take-all patch disease in Canada. Consult provincial publications for recommendations.
Grasses other than creeping bentgrass are much more resistant to take-all patch disease. No fully resistant cultivars of creeping bentgrass are known.
Dr. Tom Hsiang is the Turf Pathologist, University of Guelph and will be a special guest speaker at the 37th WCTA's 2000 Annual Conference and Show in Victoria.
Turf Line News December 1999/January 2000
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