Gardeners who like a challenge certainly got one this year. Weather conditions in Washington have caused real problems, especially for rose gardeners who don't spray their plants regularly.

Black spots and powdery mildew, two fungus diseases, have been worse than usual. Spores (seeds) of the black spot fungus are spread by wind and splashing water: Infection occurs when water remains on the leaves for several hours. With frequent rain, plants are constantly vulnerable.

Its most serious effect on the plant is early leaf loss. Premature defoliation reduces food production, lessens the plant's vigor and makes it more susceptible to other diseases, insects and winter injury. In the spring the plant may be too weak to get a good start and may die soon after leafing out. So it's important to spray regularly: once a week with a suitable fungicide -- benlate is one of the best -- during rainy periods or when overhead watering is done.

A high volume of spray is essential with each application in order to thoroughly wet both sides of the foliage.

Black spot is found mainly on leaves. But diseases occur also on the canes, and they are more likely to harbor disease from one season to the next than are the infected leaves which fall to the ground.

Research has shown that severe pruning before the plant breaks dormancy in the spring gives much better control of black spot than any dormant spraying or ground treatment.

While black spot is favored by rains or wet foliage, especially during warm weather, the incidence of powdery mildew is increased by high relative humidity close to the leaf surface. In fact, it has been shown that spores of powdery mildew germinate poorly in water but thrive at high humidity. Here again, Benlate is an effective disease control.

Many newer varieties of rose have considerable resistance to powdery mildew and some resistance to black spot, but probably none are completely immune. Most of the floribundas are in this group.

In my rose garden, the 1980 All-American rose, Love, Honor and Cherish so far have been free of infection, while several others nearby have struggled. None have been sprayed.

This is the first year in a long time I have seen black spot or mildew in my garden. If the weather next year is similar to this year, I'll have to spray or give up trying to grow the susceptible ones.