August 06, 2009
Common Disease Problems in August
Weather is always a factor when it comes to plant diseases. High humidity and cool nights are conducive to disease problems. From tomatoes to turfgrass, a range of diseases can make gardening in August a challenge.
Tomato plants are showing symptoms of gray leaf spot, leaf mold and anthracnose. While these fungal diseases cannot be eradicated once established, the progress of the disease can be slowed with an application of a copper fungicide. It's important to note that the pathogens causing these diseases exist naturally in the soil. Water droplets can splash from the soil to undersides of tomato leaves, inoculating plants with the pathogen. Keep overhead watering to a minimum and mulch plants with shredded newspaper to minimize water droplet splash. Problems with the tomatoes themselves include bacterial speck and bacterial canker, manifesting as spots 1/16 to 1/2 inch across. Few options exist to control bacterial diseases; copper fungicides can give limited control.
Rose rosette and rose mosaic are two diseases caused by viruses. Symptoms of rose rosette are stunted growth, an abundance of thorns, and leaves that stay maroon in color. Bright yellow blotches, streaks or dots on rose leaves are symptomatic of rose mosaic. Unfortunately, there are no homeowner options for eradicating these diseases. Because rosette and mosaic are vectored by insects, namely leaf hoppers and aphids, control of insect pests is essential. Infected plants should be rogued out to prevent spread to healthy plants.
Powdery mildew is a grayish white fungus of landscape ornamentals such as lilac, rose, phlox and honeysuckle. Infected leaves may yellow and drop prematurely. When making plant purchases, it's best to choose plants noted for their mildew resistance. For those plants already established in the landscape, powdery mildew can be controlled with a light rinsing of water on a daily basis. For chronic powdery mildew problems, fungicide applications can begin at budbreak in spring and repeated every other week through early summer. Cleanup of leaves in autumn will reduce inoculum to re-infect plants next year.
Anthracnose, apple scab, and various rust diseases are common problems of trees this year. Trees that have been established for 5 years and longer withstand infections readily, much more so than newly-planted trees. Newly-planted trees can have their photosynthetic capability depleted at a crucial stage in their establishment process. While it's most effective to do a preventative fungicide program, spraying once the fungus has a foothold is not effective and can be costly. For susceptible trees, plan a fungicide spraying schedule for next year.
Summer patch, brown patch and dollar spot are fungal diseases of turfgrass, primarily Kentucky bluegrass. Summer patch is a disease of the turfgrass roots; brown patch and dollar spot are foliar diseases. Preventative fungicide treatments for lawns with a history of summer patch infection may be applied in May and again in June. Dollar spot and brown patch fungicide applications as curatives are quite effective in controlling these diseases. To minimize turfgrass diseases, be sure to irrigate in the early morning hours, avoid foot traffic when turf is wet, and improve soil drainage through aeration maintenance practices.
For information on these diseases and more, contact your county Extension Office or go to Extension Publications.
8/6/09-DM Kathleen Cue
UNL Extension Extension Assistant (402) 444-7890
 
Dan Moser IANR News & Photography Coordinator (402) 472-3007
Department:
Extension
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