In the Garden:
New England
August, 2011
Regional Report

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The tiny tubes growing out from spots on the undersides of these crabapple leaves are releasing spores now that need to infect red cedars in order to continue the life cycle of the rust fungus.

A Fungus Among Us

While walking my dog one morning recently, I noticed that my neighbor's lilac bush, whose leaves are usually covered with the whitish bloom of powdery mildew by this time in the summer, was looking surprisingly good. Just a few leaves showed telltale signs of this fungal disease that commonly infects lilacs.

On the other hand, just about all the crabapples in my neighborhood look like someone took a spray gun and peppered the leaves with orange and red paint. All those spots are signs of another common fungus disease called cedar-apple rust. This year even those trees that in most years show pretty good resistance to the rust fungus are covered with spots.

Why did one kind of plant get hit hard with a fungus infection, while another kind shows only mild symptoms? Blame it on the weather. In the case of cedar-apple rust, the cool, wet weather we experienced in much of our region this year set the stage for the fungus to go to town. While many fungal problems are made worse by wet weather, there are exceptions. Powdery mildew on lilacs and other plants is a fungus that is encouraged by drought stress and is actually often less of a problem in rainy weather.

Cedar-Apple Rust
The fungus that causes cedar-apple rust has a pretty amazing life cycle (although it might be easier to marvel at if it weren't making our apple and crabapple trees look so bad!). It needs to spend time growing on a juniper as well as an apple, as its scientific name Gymnosporangium juniperi-virginianae suggests, in order to complete its life cycle.

The first symptoms on apples are pale yellow spots on the upper surfaces of the leaves and on developing fruits in mid to late spring. The spots gradually get bigger and turn orange or red, and you may see black dots in the upper surface of the spots. In mid to late summer, if you turn the infected leaves over, you'll see tiny tubes growing out of the spots. The ends of these tubes split open and curl back, releasing spores into the air.

Now these spores won't infect more apples. They need to land on a red cedar, which in spite of its common name, is actually a juniper (Juniperus virginiana) . These spores form corky, one-quarter to two inch, reddish brown galls on the red cedar -- sometimes called "cedar apples" -- that take two seasons to mature. Then, in wet spring weather, gelatinous orange-colored "horns" emerge from the galls, looking for all the world like some alien creatures that have just landed from outer space. When the weather turns dry, the horns shrink and dry up, but re-emerge in the next rainy spell. They produce spores that are carried on air currents to apple leaves and the cycle of infection begins again.

The damage to red cedars, a native plant that grows throughout our region and is also used as a landscape plant, from this fungus is minimal. But severely infected apples and crabapples may suffer enough leaf loss to weaken them, and infected fruits are small and misshapen.

Powdery Mildew
The fungus that causes powdery mildew on lilacs (Microsphaera penicullata) thrives in both humid and dry weather and most species of lilac are susceptible to some degree. It tends to be a disease of mid to late summer, causing the leaves to be covered with irregular patches of a grayish-white, powdery coating. While it can cause leaves to yellow and drop prematurely, in most cases it doesn't harm the plant so much as decrease its aesthetic value -- in other words, it looks bad. While high humidity and poor air circulation (along with hot days and cool nights) favor powdery mildew, the spores, which are released in spring from fungi that overwintered on fallen leaves, need to have dry tissue on which to germinate. So when we have a very rainy spring or summer, as we did this year, we see less of this fungus on lilac leaves, the opposite of many fungal diseases.

Preventing Problems
Powdery mildew on lilacs is generally not enough of a threat to the plants to warrant using fungicides for control. Planting lilacs in a sunny spot with good air movement, thinning out over-crowded plants by removing some of the oldest stems to the ground every few years, and raking up and destroying fallen leaves at the end of the season will usually give reasonable control. Drought-stressed plants are most vulnerable to infection, so make sure plants are watered during dry spells.

A sure-fire, but often not very practical way to control cedar-apple rust is to short-circuit the fungus' life cycle by removing any red cedars growing within four or five miles of your apple tree. Much easier is to choose resistant varieties of apples and crabs. This will usually keep problems to a manageable level. But when we have the occasional wet spring, as we did this year, even resistant varieties are likely to show a higher level of infection. Fungicide sprays can provide preventative control, but they need to be applied several times early on (when the flower buds turn pink, when 75 percent of the petals have fallen, and again 10 days later) for good control.

And if you can step back a bit from your role as gardener, take a moment simply to marvel at the the ability of nature to evolve such a intricate dance of spores in this reproductive ballet that involves two plants, three spore-producing structures, and two years to complete.


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