Springtime in the Central Valley is usually warm with few spring showers and no heavy rain storms that keep humidity levels high.Our short spring is followed by long hot, dry summers.The summer heat and lack of rain do not allow for the proliferation of most fungal problems.
The most common fungal disease problems that bring gardener’s questions to Fresno County Master Gardeners in spring are anthracnose on sycamore, oaks, ashes and elms and powdery mildew on roses as well as on the tender new leaves of summer vegetables.Treatment for both types of fungus are not easy and not very effective.The severity of symptoms will depend on the weather.The warmer and wetter the spring season, the worse the symptoms and effects of anthracnose; a mild, drier spring will contribute to powdery mildew problems.
Anthracnose disease causes the first leaves in spring to develop brown patches. On sycamores and oak trees the patches or lesions appear between leaf veins.On ash trees the patches are irregular.Infected leaves fall off, defoliating trees from the bottom up, leaving some green leaves on top of the tree.A second crop of leaves will sprout after the first have dropped and infected trees usually recover well.However, there may also be some twig and branch dieback on severely infected trees.
Chemical fungicides are preventative when sprayed on healthy tissue, but do not control or eliminate the fungus on infected trees.Some tree companies recommend root injection of fungicides but this practice is not recommended by UCCE.Spraying is not always completely effective, timing is difficult and it’s expensive.The best recommendation for control of anthracnose is regular cleanup of all fallen debris during the growing season and in winter to reduce the numbers of fungal spores that can be spread by rain or splashing water. Winterpruning directed to ensure good air circulation will also help reduce anthracnose severity.
Powdery mildew is not caused by high humidity levels in spring.It develops in mild temperatures (60 to 80 degrees) and on plants in shaded areas or in areas in the garden with poor air circulation.We see only a brief period of such moderate temperatures in spring so that the signs of powdery mildew infections are often missed until a little later in the growing season.
The first signs of powdery mildew are white spots on both upper and lower leaf surfaces.Powdery mildew spores can be washed off leaf surfaces during this first stage.As the fungus spreads, leaves turn brown and fall off and buds and new green growth become distorted.I’ve also noticed that after temperatures have risen enough to kill the powdery mildew spores, leaves on previously infected plants will show holes or areas of thin brown leaf tissue.Many gardeners will mistake these holes or brown patches for some type of insect damage.
Treat for the first signs of powdery mildew by washing off leaf surfaces and by applying horticultural oils or neem or jojoba oil.Applications of sulfur products will prevent powdery mildew problems but only if applied before the first signs of infection.Plant resistant varieties in sunnier spots with good air circulation.Sometimes the only solution for treating plants like roses which suffer severe powdery mildew infections every year is to remove the plant.