Weather conditions have been a bit all over the map lately, eh? One day it is hot enough to fry an egg on the hood of my car (very tasty by the way), and the next, I swear I see puffins slip-sliding down Christiana Creek. The one environmental factor remaining constant, however, is good old midwest humidity, due mainly to large rainfall events. Yesterday, I was able to wave my hand in the air and write my name, like on a mirror! Persistent high relative humidity means one thing: fungus among us. Oh, do not worry. There is a fungus for every condition: some like it hot, some like it cold. So, basically, we can expect a higher incidence of fungal symptoms appearing on plants in your garden. For example, the TIGER EYES sumac (Rhus typhina ‘Bailtiger’) annually develops leaf spot prior to leaf drop in late summer. It has already begun to appear, several months early. Warm temperatures, combined with high relative humidity, are the perfect conditions for powdery mildew. Do not be surprised to see a bit more of it this year if conditions continue trending as they have.
When symptoms appear on a plant, as always, it is important to first diagnose the problem by doing a little detective work. Are you dealing with biotic or abiotic causes? Are you dealing with primary or secondary causes of symptoms? Yesterday, a staff member directed my attention to some roses with yellowing inner leaves, those leaves on the bottom and toward the center of the shrub. My first reaction was to blame black spot, the curse of the rose world. Even Knockout Roses, bred to resist blackspot, can develop the fungal symptoms in weather conditions such as we experienced this spring. However, this does not seem to be the primary cause, even though black spot is present. As the picture shows, a majority of the inner leaves are yellowed with few to no black spots. Biotic symptoms such as caused by fungus tend to be more randomly distributed on a plant, unless the infection is far advanced. The plant is showing systemic symptoms suggesting something in its environment (abiotic) is causing the problem. Well, lo and behold, roses can express such stress symptoms when receiving too much water or overheating. The black spot is a secondary issue still needing to be addressed, but not the most likely cause in this instance. It always pays to not to rush to judgement.
With no end to the moisture in sight, keep your eye peeled for fungal problems to arise. Ecological outbreaks are usually the cause of either excesses (rain rain go away) or scarcity after all.
Horticulture and Facilities Manager