alternatives to pesticides - plant disease - black spot
Black spot is a fungus common to rose bushes. Symptoms appear as coal-black lesions on upper and lower surfaces of leaves. Heavily diseased leaves tend to turn yellow and drop prematurely. When excessive premature defoliation occurs, the plant forms a new set of leaves, which causes a considerable drain on food reserves in the roots. This results in a weakened plant with poorly matured wood. A few days after the spots first appear, little black pimples show up in the spots; this signals that the spores are about to be discharged and you should act fast to remove and discard those leaves. Spores are carried by air currents, insects, tools, hands and clothing.
When selecting a rose bush to buy, consult a knowledgeable nursery person or the WSU Extension agent or Master Gardeners for varieties that are less prone to black spot. Some varieties are more susceptible than others.
Remove infected leaves from the bush and those already fallen on the ground so they can’t re-infect the plant. Prune away excess foliage and sickly canes to improve air circulation. In the spring, remove the mulch around the base of the plant and rake the ground thoroughly to expose it to the sun. When new shoots appear, apply generous amounts of new mulch. Never water roses from overhead. Keep the foliage as dry as possible because dry leaves won’t support the fungus.
Least-toxic chemical control
Dust with a finely ground sulfur when temperatures are under 85 degrees. It tends to burn leaves if applied in very hot weather.