The stars have been aligned just right this spring and summer to produce a ton of disease problems in the landscape. We’ve had one of the worst years I can remember with fire blight on Bradford and other flowering pear trees. Then came a bad infestation of powdery mildew on crape myrtles. And two weeks ago, we had the new disease downy mildew that seems to be wiping out all the impatiens in the Augusta area.
Another big problem seems to be disease and death with another one of our favorite annual flowers – vinca. I think a lot of the problems started with the three or four straight days of rain we had a few weeks ago. We like the cooler, rainy weather but so do some of our plant diseases.
Vinca started in Madagascar, off the African coast. It is well adapted to hot, dry, windy slopes, bright sunshine, well-drained soil and prolonged summer heat.
The soil there has very poor nutrition, so native vinca has to be nutritionally efficient to survive. Once established in the spring, it needs no fertilizer. When planting, fertilize lightly, at perhaps half the label rate. Fertilize lightly again a couple of times over a six-week period as the plants are getting started. Then don’t do it anymore after that. If you use a slow-release such as Osmocote, a one-time fertilization at planting time is enough.
Once established, vinca requires very little water. We can’t do anything about several days in a row of rain, but most gardeners tend to overwater vinca. Most of the vinca gets overwatered because they are on the same zone of an automatic sprinkler that is watering the lawn two or three days a week, if not more. You would be better off to plant vinca in an area of the yard that a sprinkler never hits. That way, you can just hand water if and when the plants ever need it. When watering, do it early in the morning so the foliage can dry as quickly as possible
Airflow is critical with vinca. Give it room to grow. Improving air flow reduces disease problems. Proper spacing allows the soil to dry out quickly after watering or fertilizing.
There are several diseases vinca can get, and I will mention three of the most common.
The No. 1 disease is phytophthora stem blight and root rot. The symptoms of this disease are: dark brown to black lesions form on the stems and branches, causing the portions above to wilt and die back. Symptoms of root rot include yellowing and scorching of leaves, poor growth, and stunting of plants, wilting, and eventual death. Plants with reduced root systems and individual roots tend to slough off the outer tissue leaving the inner core behind.
Another fatal disease is rhyzoctonia stem and root rot. Plants affected by this turn yellow, wilt and collapse. Plants are normally stunted, their roots have brown lesions, leaves turn yellow and plants wilt even when soil moisture is sufficient. Symptoms of this disease are similar to phytophthora.
Alternaria disease causes black, greasy looking spots on foliage that start on the lower leaves and gradually moves up the plant. As the spots increase in number, the leaves turn yellow and fall off.
To help control these disease problems, plant in beds with good soil drainage. When planting, till the area or loosen the soil and add compost or other organic materials. Do not over fertilize and over water. Try to keep the foliage as dry as possible.
Remove and destroy infected plants or leaves as soon as you see them. Hot weather usually stops most of the disease progression. Rotate your flower beds by not planting vinca in the same place year after year unless you replace the soil.
If you feel you must spray with a fungicide, you can use Immunox (myclobutanil), a copper fungicide, or Danonil 2787 (cholorothalonil).
Don’t expect good results if we get a lot of rain or if you over water and overfertilize.
DOWNY MILDEW ON IMPATIENS
In my June 22 article on downy mildew in impatiens, I asked readers to let me know if they have the problem.
As of this writing, I have heard from well over 50 people by either phone or e-mail. Most said they wondered what was happening to their plants as it had never happened before.
If your plants have not gotten it yet, I am afraid it may be only a matter of time as downy mildew is a prolific spore producer. If we stay relatively dry the rest of the summer, they may not. The danger lies in several rainy days in a row with slightly cooler than normal temperatures.