Rust fungus attacks a Hawthorne fruit.

by Sue Ann Rybak

A recent walk in my backyard confirmed my worst fears – my plants and trees were under attack from fungi. Last year’s mild winter coupled with an early soggy spring has resulted in an onslaught of plant diseases and pests. My once-delicate rose petals and shiny green leaves are being ravaged by a fine, powdery, slimy mildew. And they’re not alone.

Ken LeRoy, a certified arborist who has worked at Morris Arboretum and teaches at Mt. Airy Learning Tree, said this year’s unseasonably warm weather and a mild dry winter has resulted in an increase in insects and diseases in plants.

More than 80 percent of plant diseases are caused by fungi, and this year’s record warm temperatures, high humidity and drenching rains are the perfect breeding ground for it.

“Usually a hard winter will decrease the insect population,” LeRoy said. “Plant pathogens are able to survive the winter inside insects, plant debris and soil.”

The result, he said, has been an increase in plant diseases.

Three of the most common plant diseases are anthracnose, white powdery mildew and rust.

“Fortunately, they are not killers,” LeRoy said. “Usually, if the plant is healthy enough, it is able to grow through it.”

Anthracnose disease causes dark sunken lesions on leaves, stems, flowers and fruits. It also attacks the twigs, branches and trunks of trees. It is particularly severe in sycamore, ash, oak, maple and dogwood trees. The disease is spread through microscopic spores that are carried by the wind or rain.

“The problem is by the time you see it, it’s almost too late to do anything,” LeRoy said. “It’s difficult to control once symptoms appear.

While dry weather is the best control for this disease, LeRoy said there are some things you can do to combat the problem. Among these are raking and disposing of fallen leaves and other debris during the growing season and in the fall, pruning plants during the winter to increase air circulation and removing infected branches as they form to prevent trunk canker formation.

“The most important thing you don’t want to do is apply a high nitrogen fertilizer,” LeRoy said. “It will only stimulate succulent branching.

Powdery mildew is one of the most common plant diseases. Almost no plant is immune. The white or grey powdery substance can be seen on leaves, stems, flowers, buds and even fruit.

Rust is another common plant fungus characterized by reddish or brownish spots on leaves, stems, and other parts. Rust effects crabapple, apple, hawthorn and other ornamental plants in the rose family.

“Rust disease is a very sophisticated organism, and the conditions right now are perfect for rust,” LeRoy said.

Rusts tend to be very host specific. For example, the rust that attacks antirrhinum is a different type from one affecting hawthorn. LeRoy said some rusts such as the cedar-hawthorn require two hosts to complete their life cycle.

He noted that cedar-hawthorn rust causes discoloring and yellow to nearly black spots on the leaves, fruit, petioles or new twigs. He said these spots contain black pimple-like fruiting bodies that produce spores that are carried by the wind and rain.

“You can interrupt the cycle in the late summer when it starts going back to the juniper,” LeRoy said.

“I always keep neem oil in my bag of tricks.

Pure neem oil is derived from the seeds of the Indian neem tree. It is naturally biodegradable and is harmless to humans, animals and beneficial to insects such as lady beetles and bees. It is a botanical pesticide and fungicide. Unfortunately, it does not provide a quick fix nor does it permanently kill insect pests or fungal diseases.

The reality is that once anthracnose, powdery mildew or rust become established it is difficult to control with any fungicide. It is a good prevention tool when applied in the late fall and early winter once a month.

LeRoy said the key to any disease control program is proper plant placement, good plant “hygiene” and preventive maintenance.

Rust and other fungus diseases are a problem in areas where plants are exposed to moisture for long periods of time.

LeRoy said increasing air circulation around and through a plant would help it dry faster. Removing dead or infected leaves, branches or fruit will help decrease the spread of the disease. He said spraying the top and underside of the leaves and branches during the growing season would help to stop the spread of the disease and effectively treat the plant.

For more information about anthracnose, rust or other fungus diseases, LeRoy suggests contacting Penn State Extension at

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