In the last few weeks we've had intermittent rain showers with moderate or warm temperatures that have given us several weeks of perfect weather — just perfect for fungal problems, that is.
We're seeing more powdery mildew, fireblight and black spot than usual this spring. Our summer can begin in late April with high temperatures that kill many fungal spores, but we have a few weeks ahead of us before the heat arrives. Extended warm, rainy weather will exacerbate fungal problems and we'll need to use control methods to reduce fungal damage.
Powdery mildew proliferates when daytime temps are within the 50- to 70- degree range. The fungus really does look like a powdery coating on leaf surfaces and new buds. Roses, crape myrtles and sycamores are especially susceptible to powdery mildew. Powdery mildew spores can be washed off plants; do that in the early morning to allow the leaves to dry. When the spores land in water, they die.
If washing off the spores doesn't slow spore growth, use neem oil (it acts as both a preventative and an eradicant fungicide), jojoba oil or horticultural oil. Sulfur-based fungicides are a traditional treatment, but they are caustic, need to be reapplied consistently every seven to 14 days, and can damage plants if applied when temps are above 90 degrees.
Fireblight bacteria infect members of the rose/apple (pome) plant families in spring as temperatures warm. Again, the name fireblight is descriptive of its appearance. New, small twigs and branches turn black and twist; looking as though they'd been scorched by fire.
Protect susceptible plants by spraying a fixed copper solution when the combined daily temperatures are above 120 degrees; for example, 75 degrees during the day and above 45 degrees at night. The bacteria move from infected wood to the flowers in spring. Prune out infected shoots during the dry season to remove sources of the bacteria and to create good air circulation.
Black spot fungus causes black spots to form on affected leaves. The leaves turn yellow around the spots and fall off. Good air circulation, through the interior of the plant structure as well as around the plant, will help prevent black spot. Remove affected leaves as soon as the first sign of black spot and treat the leaves with neem oil or a mixture of bicarbonate of soda and horticultural oil.
Always dispose of diseased or infected leaves, stems, twigs and branches in the green waste bin; temperatures in a compost pile are seldom high enough to kill all fungal spores and bacteria. Clean up all plant litter around affected plants and trees regularly. Replace severely affected plants with resistant varieties. And, if bees are flying, wait to spray until they've gone home.