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  Fungus Among Us
  March 30th, 2007

The Unwanted Trio :
Powdery mildew, rust, and blackspot. For rose growers, these three characters are hard for us to avoid. Morning and evening air moisture will get us every time, no matter how careful we are about giving our plants the best cultural environment that we can. Oh sure, there are others! But we’ll start with these three guys.

Powdery mildew appears as a superficial white or gray powdery substance over the surface of leaves, stems, flowers, or fruit of affected plants. These patches may enlarge until they cover the entire leaf on one or both sides. Young foliage and shoots may be particularly susceptible. Leaf curling and twisting may also occur with this fungus. Severe powdery mildew infection will result in yellowed leaves, dried and brown leaves, and disfigured shoots and flowers. Although it usually is not a fatal disease, powdery mildew may hasten plant defoliation and the infected plant may become extremely unsightly. On roses, uncontrolled powdery mildew will prevent normal flowering on highly susceptible cultivars.

Some powdery mildew, especially those on roses, are favored by high humidity. This can happen in our gardens when we have plant overcrowding and shading will keep plants cool and promote higher humidity. These conditions are highly conducive to powdery mildew development.

Rust is another fungus presenting problems in our gardens. It first appears on the undersides of leaves and other plant parts as orange powdery “pustules”. As these pustules develop, they become visible on the upper leaf surfaces as orange brown spots. Rust can develop when temperatures are 65 to 70 F, and moisture is continuous for two to three hours.

It is very important to remove and destroy the infected foliage containing rust. Wear gloves that can be washed afterwards and clean any tools used in the removal. This fungus is easy to spread. That is why it is important to also clean up any foliage that has fallen to the ground under the infected plant. Replace any mulch present with new mulch. Don’t try to “wash” the rust away from the foliage! This will only help it to spread further in your garden.

Blackspot , also a fungus, appears like its name. It also develops during warm but wet weather. Unfortunately, it can overwinter in the leaf buds and canes or on fallen leaves not cleared away from your roses. Lots of sun, good air circulation and healthy soil will increase your rose plant resistance capabilities. As with rust, it is very important to remove and clean up infected foliage. Remember to clean your tools between cutting on infected plants.

Before using fungicides you should attempt to limit powdery mildew and rust by following good cultural practices.

Purchase only top-quality, disease-free plants of resistant cultivars and species.

Prune out diseased terminals of woody plants, such as rose during the normal pruning period. All dead wood should be removed. Remove from the surrounding soil all dead leaves that might harbor the fungus.
Keep plants healthy. Plant where the plants will obtain a minimum of 6 hours of sunlight daily (especially roses), space for good air circulation.
Water thoroughly without over- watering. Don’t use overhead watering/sprinklers, which wet the foliage. Don’t water in the late afternoon or evening when the foliage will not have time to dry.
Fungicides may become necessary to achieve acceptable control. For best results with fungicides, spray programs must begin as soon as mildews are detected. Ask one of us which of the fungicide products are best suited for your needs. There is a range of products available on the shelves of you local garden center.

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