In the past few weeks I have written about beetles eating holes in the foliage and sawflies devastating roses.
In addition, I am also seeing considerable black spot on roses. This is a disease that thrives under the right conditions of moisture, humidity and temperature, which we’ve had recently.
It is also a disease that develops quickly under these conditions, infecting a plant within a day and causing symptoms to show up in four to five days. Therefore, it’s a tough one to control if we continue with this wet weather pattern.
Fungicide sprays have to be on the leaf surface prior to infection. There are systemic fungicides available on the market, and these would be necessary to prevent continual reinfection of new growth.
Another major disease problem that is occurring on bedding plants, particularly petunias and vinca is phytophthora root rot. This is also due to the wet weather.
Symptoms include a general overall wilted look to the plant and possible yellowing of the foliage.
When gardeners see a wilted plant, the first thought is to water. However, it’s possible for a plant to be completely wilted with the soil wet. If there is no oxygen in the soil or if there is a problem with the root system, the plant can’t absorb water, thus the wilting.
When gardeners see a yellowish plant, the inclination is to fertilize. However, this isn’t the answer because if the roots are compromised, they can’t absorb fertilizer.
Phytophthora destroys the root and/or crown of the plant. When this occurs, water can’t be moved from the roots into the foliage, despite the fact that there is plenty of water in the soil.
A good diagnostic tool to help determine if the plant has phytophthora is to pull the plant and examine the roots. They eventually turn black and look rotted.
In addition, if you gently pull on the root and the outer layer of the root comes off, leaving a very thin piece of the root (called a “rat’s tail”), it is likely phytophthora root rot.
Plants may even be affected by the disease and may be holding on, looking pretty good. However, when warm weather hits they might collapse if their roots are affected.
Plants wilt and eventually die if they are affected by this disease. If this occurs to your plants, the best thing to do is to remove them and replant with something else other than petunia and vinca.
Geranium and snapdragons are also affected by phytophthora and should not be planted in the same area. I am seeing symptoms on my snapdragons but have not observed it on geraniums.
Next week's topic: tomato woes.
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