To everyone outside the farming world, deep, persistent snow cover is associated with little more than long, bitter, inhospitable winters, but snow shows up in agronomic contexts playing the same role again and again – as the perfect insulator. Plants are more likely to survive the winter under snow cover, and the buffered soil frost heaves less, helping the roots stay anchored. For as long as snow sticks around, it keeps a cool, mostly steady temperature and moist environment, perfect for the select few common fungus species that are adapted to colder temperatures and are opportunistic enough to feed on living species without lots of competition from other pathogens.
The snow mold fungus emerges
The warmer fall we’ve had this year (in Southern PA, where I’m writing from) was encouraging to many who had planted winter annuals – plentiful fall growth is a plus for winter survival and is exciting because the status quo seems to favor the alternative. We have to be careful with this scenario, however, since excessive growth in any crop prone to matting is perfect for creating damp, just-above-freezing conditions under the canopy of plants and snow. The poor air circulation, moisture, and live plant material give the fungus the shelter and food source it needs to survive.
Crops like annual ryegrass in particular are prone to matting, because their high proportion of leaf to stem material, despite making the plant highly digestible, also provides less structure. However, many over-wintering crops could yield to the weight of snow if there’s enough of it. Cereal grains like rye or triticale, in addition to many pasture grasses, can be affected as well, since early growth stages of these crops are also lacking in the structural lignin they produce as they mature.
With long stretches of snow cover and temperatures just above freezing, the disease can become a serious threat as it infects the upper leaves first, eventually spreading to and killing the crown. This is the point at which it does lasting damage and the plant cannot recover. There are many other instances, however, when leaves may appear damaged in the spring, but overcome this setback during the rapid growth flush of spring greenup.
Snow molds are generalists – they don’t need specific hosts to survive – but usually prefer grasses. The conditions that set the stage for the fungus to develop include deep snowfall early in the season that insulates the ground and prevents it from freezing. The plant is also weakened and more vulnerable to pathogens in the dark, humid microenvironment where faltering photosynthesis cause it to deplete carbohydrate and protein reserves through respiration. Snow molds seize the opportunity the colder environment presents, where they will meet little competition from other pathogens looking to the plant as a host. Insulation from the snow also gives the fungus a more hospitable environment by reducing the temperature fluctuations. This microenvironment traps warmth and moisture, and limits air circulation.
Snow cover is not technically necessary for the molds to develop, but it does often create the ideal conditions. A thick layer of matted leaves can do the same thing. However, different molds need more insulation and consistent temperatures than others. Gray snow mold, for example, is more dependent on long-term snow cover than pink snow mold.
The fungal growth can take place at temperatures slightly below freezing and continue through snow melt, as long as grass stays cool and wet. Soil texture has no effect, except if it contributes to poor surface and subsurface drainage. Higher soil pH (above 6.5) helps the pathogens.
Plants that are already weakened by disease or weed pressure will be more susceptible to snow mold. Weeds present an additional challenge, since they can also host the disease.
The fungus first develops by attacking the longest leaves in contact with the ground. Scouting should begin in areas where snow is deepest (more than 1 foot is most likely). Damage will be apparent when snow first melts, and may appear as pink, fuzzy growth on dead or dying leaves. Or you may see spores or plant discoloration.
The “Goldilocks” amount to leave
You can monitor growth in the fall and take a late cutting if needed – generally if plants are taller than 8 inches going into winter. You can get away with about 11-12 inches with small grains, since it takes more weight for them to lodge. It’s good to leave about 4 inches at last cutting, though, to allow for good regrowth and root energy stores over winter. At this point in the season, if you feel growth is excessive, it may be most appropriate to just clip the stand lightly. It’s still better to err on the side of more growth than less, since more means better resilience and recovery in spring (yes, it’s a little ironic that the problem – excess growth – can also be part of the cure). It’s very important not to overgraze in the fall, as well, since your stand counts on the recovery time for roots to regenerate before winter. To a certain extent, higher stubble means healthier pastures going into winter. You will need not too much, not too little, but the perfect (“Goldilocks”) of residual. This perfect amount will fall within a range, and what exactly that range is, will of course be determined by the weather conditions and the costs and advantages of the increased management of the crop.
Accounting for the risks
Lots of moisture and heat in the fall gives you more risk of overgrowth. Small grains can often do better under these conditions, since more fertility and moisture cause more tillering and weed suppression, not just the taller growth that would make them vulnerable to lodging and matting.
Rotating to legumes is a good plan, since the fungus flourishes with grass as a host. Breaking up the rotation with legumes decreases the longevity of sclerotia and inoculum in the soil. No-till practices can help as well, since many spores that survive in the soil are blocked from spreading to plants by the layer of residue over the soil surface.
Fungicides are sometimes used on areas with chronic disease pressure, but again, crop rotation is a more effective strategy long-term.
Understanding what species are most at risk is also paramount. Again, ryegrass has a less stemmy structure – good for digestibility, but bad for standability – so it is best to include it as part of a mix with other grasses and legume species. Fall and winter cover crop mixes can also be vulnerable, since they are often planted earlier in the fall to get good growth and ground cover, and are likely to be a little taller going into winter.
In most situations, you can get away with a late fall clipping or light grazing to open up the stand a little bit (wait until soil conditions allow).
Despite the risk that comes with early planting, it’s still advisable in most years to be timely. Early is almost always a better bet than late – even with the risk that you may get the lush growth that comes with a warm, wet fall. Any winter annual that is planted too late runs the greater risk of not developing an adequate root system, which would leave it vulnerable to frost heaving, wet soils, and crown exposure during a cold snap. You never know what kind of fall you will get, so it does pay to take the chance of erring on the early side with planting. If conditions are ripe for snow mold, the crop also stands a decent chance of pulling through without much damage – and a better chance if you opt for a mix. So – the confusing lesson here is that although early seeding is part of the problem, it can also be part of the cure. Larger plants are more resilient, and can bounce back better in the spring.
And rotate, rotate, rotate. The longer the rotation, the better, since fungal spores can survive in residue that was previously infected.
Note: This is intended to give a holistic picture of how to manage the cropping system to avoid snow mold, and what to watch out for as the snow melts and spring approaches. It is not recommended at this late date (now December) to cut or graze, since the stand is unlikely to have a chance to recover before it goes into dormancy. In other words, the risk of damage by snow mold is less than the risk of freezing damage if the stand is too short going into winter. In addition, although the weather was abnormally warm this fall, many areas also had abnormally dry stretches and as a result may not have the excess growth that would create conditions ideal for snow mold. (Plus the obvious question of whether we will get enough snow to make this discussion relevant at all.) This will be heavily dependent on your region and the weather you received.