Q. My lovely Japanese maple has white spots on the majority of the leaves. Although it looks OK otherwise, I am concerned about these spots. They appear to be drying up the leaves. Can you tell me what is causing this and how to prevent it in the future?
A. The very wet weather we had this spring when plants were leafing out created very favorable conditions for a variety of fungal diseases to develop on all kinds of trees. Tender new growth is far more susceptible to infection than mature, hardened-off foliage.
Based on your description, it sounds like anthracnose. This is a very common fungal disease that attacks maples, dogwoods, sycamores, oaks and other species. To complicate matters, the disease has different causal organisms on different species, and the expression of anthracnose can vary, depending on the causal organism.
Aureobasidium apocryptum is one fungus that causes anthracnose on maples. On Japanese maples, this anthracnose appears as discreet white spots. As the disease progresses -- and the longer the leaves stay wet, the worse it will be -- the spots generally run together and kill larger areas of the leaf, resulting in a scorched appearance. On occasion, this anthracnose can cause twig dieback, too, especially on succulent shoots.
Although it does detract from your tree's ornamental appeal, anthracnose on maples is not life-threatening. Fungicides are best used to protect new growth as the tree is leafing out in spring. Nothing you can apply now will "cure" those spots. They will remain until the tree loses its leaves this fall.
The best practice is to clean up the fallen infected leaves this fall. Send them out with the trash. If the disease has resulted in dead twigs, prune them out in late winter and get rid of them, too. Treatment is not usually recommended because anthracnose does not cause significant damage to maples. If you cannot tolerate any damage, apply a fungicide containing mancozeb at bud break and at seven- to 10-day intervals until wet weather stops and average daily temperatures top 65 degrees.
It also helps if the tree is growing in full sun and is not crowded by other plants. This permits good air circulation and rapid drying of foliage after it rains. If this is not the case, perhaps you can prune the surrounding plants to permit better air movement around the tree. Not that it would have helped much this spring; the plants rarely had a chance to dry off in between storm systems.
Q. I used to grow radishes with no problem. Lately, all I get are plants, no radishes. What is causing this?
A. Radishes fail to bulb for several reasons. When seeds are sown too thickly and not thinned properly, there may not be enough room for the crowded plants to form bulbs. Radishes should be thinned when plants are 1 to 2 inches tall so they are about an inch apart.
They also fail to bulb properly when temperatures are too hot. If they mature when temperatures are above 80-85 degrees, they often bolt to seed instead of bulbing. This also happens if they are grown in too much shade.
Improper fertilization can also cause this problem. Too much nitrogen -- whether from fertilizer or manures -- forces plant growth rather than bulb formation. Fertilizer for radishes should have low nitrogen and higher potassium (potash). The numbers on a container of fertilizer such as 5-10-10, refer to the percentages of nitrogen, phosphorus and potassium in that product. They are always listed in the same order.
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