I had a call this summer about a jasmine bed dying in a big circle about 20 feet in diameter. No, it was not alien spacecraft engines burning the jasmine and not raccoons making crop circles.
It was Rhizoctonia aerial blight. The jasmine bed was fine for the past six or seven years, but then started to die last year and got worse this year. The death of the jasmine coincided with the return of rainfall. The bed also has an overhead irrigation head in it, and the homeowner assured me the irrigation works.
Rhizoctonia is a fungus in nearly every yard since it has a wide host range as a beneficial mycorrhizal partner for trees. Mycorrhizae absorb water and minerals for trees so well that trees and other green plants (with just a couple exceptions) are totally dependent on these fungi for their water and minerals.
It is not a slobbering wet kiss kind of relationship between the Rhizoc and the trees. The tree tries to throw off the fungus and the fungus tries to infect the tree root tips. It is kind of like trading with the Chinese. Both parties can benefit greatly, but only if they keep a watchful eye on each other.
The moist cool nights, ample rainfall and extra irrigation boosted the vigor of the Rhizoc and held back the jasmine just enough to give the Rhizoc the upper hand. Early in the morning the infected jasmine will have a thin webwork of fungus strand glistening with dew. One has to be there before the dew burns off in order to see it.
How to stop it? Return the balance of power to the jasmine by modifying the environment of the jasmine — reduce water. While we cannot control the rain, we can shut off the irrigation system. Do what pruning one can of surrounding plants to increase air circulation to further reduce the wet period.
After that, there are some fungicides that will slow down the fungus enough (not kill it) to give the plant a chance to reassert itself. The same fungus does the same type of thing on lawns in the spring and fall. We call it “Rhizoctonia large patch” or “brown patch” when it hits a lawn. It is the most submitted disease sample to UGA Extension Office state wide, year after year.
We agriculture agents are not a particularly imaginative lot. If we see an aphid that is white and wooly and it is feeding on a rose, why then it obviously is a white wooly rose aphid. If that same aphid crawls over onto a sand pear tree next to the rose then it obviously is a white wooly pear aphid.
Rhizoctonia makes large brown patches of grass it infects in the spring and autumn, so it obviously is “Rhizoctonia large patch.” When the very same fungus grows onto the aerial parts of plants when humidity is high and the sun cannot reach it and kills leaves, stems and flowers, why it obviously is Rhizoctonia aerial blight.
Maybe the best example of calling-it-what-it-looks-like is slime molds. Yep, you guessed it. Slime molds often look slimy and are a mold (fungus). They tend to show up in mulch that has not composted long enough before being put out into the landscape.
When this happens in highly visible public places is tends to grab attention. I was once called out to diagnose what happened to allow a particular slime mold to show up in shrub beds at a resort. I told the grounds manager that it was not something to spray. It would go away with the next rain.
“So what is this called?”
“I can’t tell my director that, and that I am not going to spray it. Does it have another name?”
“Yep, dog vomit fungus.”
“You are not helping me, Don.”
Like I said, no imagination. We call it what it looks like.
Back to the point at hand — and yes, I hear you thinking, “Please Don, let there be a point” — over the past seven or eight years we have been through a severe drought. Do you still remember it? Most people seem to have forgotten all about it.
Trees go through as many as nine steps in sequence to shut down in order to survive a drought. They have to go back through those steps in reverse order before they have fully recovered from a drought. It can take as many as two years for a tree to fully recover. Even though the rainfall has returned for almost a year, many trees are not fully recovered and are still at risk for decline and death because of the drought for at least another year.
There is a natural sequence to these things. If a fire sweeps across the African savanna and burns off the grass, the population of antelope crashes, followed by the lion population. Once the grasses return, the population of antelope can rebound. After the antelope population rebounds the lion population will return. Until the lion population returns there may be some overgrazing of grass as the antelope population explodes due to a lack of lions predating the antelope herd.
The same thing has happened in Yellowstone with the reintroduction of the wolf population. Sycamore tree populations in the park were declining steadily until the wolves were reintroduced. Now the sycamore population is growing. With no predators, the elk and moose were free to munch on sycamore all day long and overgraze it. With the return of the wolf, the herbivores have to keep a wary eye out and do not eat as much sycamore as they used to. The population of sycamore returns as the things that eat the things that eat the sycamore return. Not only are dogs man’s best friend, in Yellowstone wolves are sycamore tree’s best friends.
As our plants make a comeback from this most recent drought, the pests that compete with them or consume them, be they plant, insect, fungus or bacteria, are making a comeback also. I am seeing explosions of weed populations (and not just in my yard either), insect populations and fungus diseases across the board. I have noticed this happening in traditional agricultural crops, landscape plantings, trees, lawns, household pests, just about everything.
The predators on these pests have not rebounded yet so the explosion of tent caterpillar is to be expected. When the predators of tent caterpillar return in future seasons, the density of tents will decline. Like a rock lofted into a pond, the first ripple is the biggest.
So the point is, all this explosion in pest activity is not the sky falling. It is just populations of the food web and predator/prey relationships adjusting to the return of normal rainfall. Natural. Predictable. Pretty cool if you are into population dynamics. Not so much fun if you are trying to produce food and fiber. And no reason to start spraying poisons all over the home landscape. This too will pass.
Gardner lives in Keller and is the UGA extension agent for Glynn County, serving South Bryan.