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The Garden Plot for June 22, 2007
3:55 PM - 4:00 PM
[Program Website]
Today's Highlight: Disease Time II
"The Garden Plot," June 22nd, 2007: "Disease Time II"
Scabs, cankers, and blights - that's what we're seeing in the Missoula County Extension Service plant clinic now. These diseases showed up after the warm, rainy weather we had in late May and early June. When weather is wet and humid and temperatures are 60-70 F, many fungal and bacterial diseases are most infectious. Like plant seeds, fungi and bacteria need a wet surface on which to germinate. When water sits on leaves and twigs for several hours, disease occurs. Windy, wet weather spreads disease from leaf to leaf and plant to plant. The most effective disease management method is to keep irrigation water off of plant leaves and stems. When our weather gets dry, the only way diseases will spread is by irrigation water.
Sunken cankers on branches, blackened new twigs, and dark blotches on apple and crabapple leaves may be caused by a bacterial disease called Fireblight. We had a high Fireblight infection period three weeks ago. Prune out blackened twigs on a hot, dry day. Sterilize your pruners with bleach in between cuts. If more then 10% of branches are affected, you might want to spray with copper after pruning. Spray in the evening when temperatures are below 75 F to avoid burning leaves.
Blackened new twigs, dark spots, and blotches on aspen leaves may be a symptom of aspen blight, a fungal disease. There are also other fungal leaf spot diseases that attack aspen, causing dark to orange-yellow leaf spots. Protect new leaves by spraying with sulfur if weather is wet and warm. Rake up and destroy diseased leaves when they fall.
Another fungus disease is causing dark-colored lesions on cherry and apricot leaves and fruit. Shot-hole fungus disease likes hot, wet weather. It also causes small purplish-red spots on cherry, plum, and apricot leaves. Protect leaves and fruit with a fungicide spray. Sulfur is the least-toxic option.
White, powdery leaves and twigs at the ends of apple, crabapple, and carragana branches, are caused by the fungus called powdery mildew. Prune out infected terminals on a hot, dry day. If humid, wet conditions persist, you can apply preventative sprays, including Neem and products that contain baking soda, such as Remedy.
Apple Scab is causing velvety, brown spots on apple leaves. Later infection periods can infect apple fruit causing dark spots and cracks associated with the expanding spots. If you see apple scab spots on apple leaves and the weather stays wet and warm, you may want to protect leaves and fruit with a fungicide spray. Sulfur is the least-toxic option.
In summary: Diseases like wet leaves. Keep water off of leaves, stems, and branches. If you must use sprinkler irrigation, irrigate after 10 am and before 3 pm to maximize leaf drying conditions. Also, create the best leaf drying conditions possible. Prune out branches to maintain good air movement.
Call the Missoula County Extension Pest Alert phone line at 258-3820 to find out if and when disease infection periods are occurring.
Helen Atthowe's new short program of gardening tips
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