Turf Disease Or Weather?

“When it rains, it pours.” And it has brought lots of issues with our yards other than slime mold. Even growers are asking about these issues they are seeing. Why is the grass is turning yellow and dying in irregular shapes? In between our commercial ag calls, us county agents answer residential calls about landscape issues also. I’ve learned a lot about centipede and St. Augustinegrass in 3 1/2 years, but some of the things I’m seeing for the first time this week.

Rain and humidity provide the environment for disease. Usually, Large Patch (Rhizoctonia solani) will show up in centipedegrass during this time. You will see brown spots in the lawn with a nearly perfect expanding circle. St. Augustine generally gets Take-All Patch (Gaeumannomyces graminis) with significant damage. Another common issue during periods of rain is yellowing of the grass blades. This is result of nitrogen and potassium leaching out with rain (below).

Centipede nutrient deficiency from rain

Centipede nutrient deficiency from rain

The reason I wrote this post is because I visited a centipede lawn earlier this week with dead grass in irregular shapes. The symptoms did not look at all biotic (disease, insect damage) but abiotic (environmental, cultural). The lawn held lots of water and you could clearly see where water sat for long periods, and roots went without oxygen and died. However, I saw a few small circles, but not typical sign of Rhizoctonia. I took some samples and looked under the microscope. Under the dissecting scope, when you see these black dots…. they are usually hyphopodia (puzzle pieces) from Take-All Patch…

Centipede-Dissecting

Mycelium on base of centipedegrass blade

……however, we always look under the compound scope to confirm it. In this case, the “hyphopodia” turned out to be clumps of mycelia. The issue was that these mycelia did not have all the key ID features of Rhizoctonia (Brown color, crosswalls, 90 degree branch angle, and taper at the branch).

Centipede-Middle.jpg

Clumps of mycelium on centipedegrass

Below is actual key ID feature of Take-All Patch…

Hyphopodia from Take-All Patch on St. Augustine

Hyphopodia from Take-All Patch on St. Augustine

I sent to UGA Pathologist Dr. Elizabeth Little who responded saying sometimes the mycelia will clump as it enters the cell, but she and her diagnostician was not convinced this was Rhizoctonia. Below is the response from Dr. Little. To summarize, she believes the weather conditions are causing lots of issues with lawn now, and Rhizoctonia found in the samples is secondary:

I showed this to our diagnostician who sees lots of Rhizocontia and she was not convinced this is Rhizoctonia. She thought it was too erratic. However, the runner hyphae of rhizoctonia will make infection cushions of balled up hyphae where they enter the plant. That is what these look like to me. So this could be a random saprophytic fungus or possibly Rhizoctonia but you would need to look some more to confirm. When conditions are wet like this Rhizoctonia and other fungi are everywhere. I am getting many centipede samples of poor greenup but I am thinking any Rhizoctonia is probably secondary to the environmental stress, especially if there is no distinct pattern of large patch.

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