Research News x Fungus that causes dollar spot

from News Notes - Fall 2021 - Vol 1 Issue 2

News Notes - Fall 2021 - Vol 1 Issue 2

Fungus that causes dollar spot

By J.M. Vargas Jr. Professor, Dept. of Plant, Soil, and Microbial sciences Michigan State University

Introduction

The name of the fungus that causes dollar spot has been changed to Clarireedia jacksonii from Sclerotinia homoeocarpa. There are three other species that are responsible for dollar spot throughout the world: Clarireedia monteithiana, Clarireedia homoeocarpa, and Clarireedia bennettii. However, Clarireedia jacksonii is responsible for most of the dollar spot on cool season grasses throughout the world including Michigan.

Folk Lore

I like to say turfgrass science has a lot of what I call “folk lore” still in it. This describes beliefs we have about turfgrass management that are based on observations, greenhouse experiments, and things that seem to make sense but do not have replicated research under field conditions to back up these claims. Some of this is because turfgrass science is relatively new compared to research done on field or fruit crops which has been going on for over a hundred years so they have eliminated most if not all of the folk lore. Most of the critical turfgrass research began in the 1960’s but it didn’t really become wide spread at many university until the 1980’s. Maybe the all-time classic folk lore was that Poa annua died in the heat. Of course we now know based on field research that it dies in the warm weather from disease and insect problems.

Dollar Spot Irrigation

Dollar spot has some folk lore associated with it some of which I am personally guilty of. In the three additions of my text book “Management of Turfgrass Diseases” I tell you the worst time to water the turf is in the early evening because the turf will remain wet for a longer period of time thus resulting in more disease. So at Michigan State University we set up an experiment where one set of plots received a .10 inch daily irrigation at either 10 pm or at 5 am for a total of .7 inch a week. A third set of plots received the same amount of irrigation .7 inches a week applied twice a week which represented a deep infrequent irrigation program. The results showed a pm daily irrigation had the least dollar spot followed by the daily 5 am daily treatment with the deep infrequent program having the most dollar spot. We believe the reason the daily irrigation programs had the least dollar spot was because of the moist soils supporting higher levels of bacteria which were antagonistic to the dollar spot fungus. We know moist soils can support high populations of bacteria and that bacterial • populations are lower in drier soils. Also included in the study were 3 creeping bentgrass cultivars: L-93 a susceptible cultivar, Declaration a moderately

resistant cultivar and Flagstick a highly resistant cultivar to dollar spot. Whereas Declaration and Flagstick had the least disease and L-93 had the most dollar spot regardless of irrigation regime all the cultivars had less disease in the daily irrigation plots compared to the twice a week irrigation regime. This is why you need to do the experiment under field conditions and not base recommendation on what sounds good or seems to make sense.

Rolling Greens

There was great concern that rolling greens would cause compaction resulting in poor quality turf. This may have been true if we still had soil greens. Research at Michigan State University showed there was not enough compaction to injure the turf. A surprising result of the study was that there was less dollar spot in the rolled plots. In an attempt to explain the reduction in dollar spot in the rolled plots we examined the soil moisture in the upper 1.5 inches of the greens. The research did show there was more moisture in the upper 1.5 inches of the rolled green suggesting the rolling had reduced the macrospore space and produced more microspore space resulting in greater moisture holding capacity in the rolled plots. Further research showed there were high levels of bacteria in the rolled plots compared to the non-rolled plots.

Other well-known cultural practices

Other cultural practices that we have known about for a long time are also important in managing dollar spot. Removing the dew from the greens as soon as possible in the morning stops the fungus from continuing to spread. Of course what we are really doing is removing the guttation water that was produced by the plant which contains nutrients that help support the growth of the fungus. Adequate nitrogen fertility will also reduce the severity of dollar spot by having the plant growing so infections can be mowed off before they develop into a lesion and new leaves can be produced to replace those infected by the fungus.

Conclusions

In conclusion night time irrigation does not increase dollar spot and if done on a daily basis will result in a decrease in dollar spot. Rolling greens will also result in a decrease in dollar spot by supporting higher levels of bacterial populations. This is related to the increase in microspore space in the sand which causes an increase in moisture levels in the top 3.8 cm of the green. Removing the guttation water as soon as possible in the morning and having adequate levels of nitrogen to increase the growth of the plant will also reduce the amount of dollar spot. Finally there are now creeping bentgrass cultivars like Declaration and Flagstick with resistance to dollar spot which will also help reduce dollar spot especially when combined with the above mentioned cultural practices.