Lexington, Kentucky 40546
      
Nursery Update - A University of Kentucky Cooperative Extension Service update for the Kentucky Nursery Industry
 
By Amy Fulcher, Extension Associate - Nursery Crops
 
University of Kentucky Department of Horticulture
 
Update #13
 
June 3, 2003
 

Disease:
Powdery mildew
Powdery mildew has become a problem on dogwoods in recent weeks. It can also be a problem on serviceberries and crabapples. Often the disease begins as barely distinguishable reddish brown or purplish irregular blotches which develop into dark brown to tan dead patches. Powdery mildew is most easily recognized when the whitish powdery mycelium is present on leaf surfaces. Other less recognizable symptoms include leaves with a mottled yellowing with brownish patches and/or new growth that is distorted and curled. Many of these curled leaves may also be scorched with brown leaf margins and interveinal dead patches.

Powdery mildew reduces overall vigor, slows growth, and reduces flowering the year following a serious powdery mildew infection.

Fungicides such as Bayleton, Strike, Systhane, or Cleary's 3336 should be applied with an airblast sprayer every 14 days through September, every 7 days if the disease is severe. Use the higher rate now that the disease is present. In UT studies in 1999 Banner MAXX, Systhane, and Rubigan AS provided the best control. In 1998 researchers observed up to a 50% increase in caliper and height where fungicides were sprayed every 2 weeks from May 28-August 19. Rotate fungicides.

Source: Dogwood Production Guide, UT publication; Dogwood Powdery Mildew Update, Kentucky Pest News Article 925, July 16, 2001; Dogwoods for American Gardens, UT PB 1670.

Range of Dogwood Resistance to Powdery Mildew

Susceptible: All Cornus florida, seedling wild types (but individuals vary in susceptibility) and most C. florida cultivars.
Intermediate susceptibility: C. florida 'Cherokee Brave' and cultivars of the C. florida x C. kousa hybrids.
Resistant: Three new powdery mildew - resistant C. florida cultivars have been developed by the TN Ag. Exp. Station and are now available in the nursery trade. They include 'Jean's Appalachian Snow', 'Karen's Appalachian Blush', and 'Kay's Appalachian Mist'. Also resistant: Cultivars of C. kousa.
Immune: C. mas.

Insect:
Borers
Peachtree borer should be sprayed for 1 week after oakleaf hydrangea is at full bloom. Flatheaded appletree borer should be sprayed for when oakleaf hydrangea is at full bloom. Lessor peachtree borer should be sprayed for when it is 2 weeks past kousa dogwood 1st bloom. These plants are at or near these stages in West Kentucky. A study on Southern pine borer, considered by many to be the acid test for an insecticide, showed that chlorpyrifos at 1% or 2% was effective for 4 months at controlling southern pine borer. A 2% spray of chlorpyrifos was effective for 10 months. This is based on the number of successful borer attacks controlled to 88% or better. See Nursery Update #12 for chlorpyrifos sources. Spray coverage should be thorough with particular attention paid to the graft union or rough bark. Nozzles with a coarse spray are most effective.

Source: Brady, U.E., C.W. Berisford, T.L. Hall, and J.S. Hamilton. 1980. Efficacy and persistance of chlorpyrifos, chlorpyrifos - methyl, and lindane for preventative and remedial control of Southern pine beetle. J.Env. Ent. 73(5):639-641; Assistance from Dr. Don Booth, Entomologist, Bartlett Tree Experts, is appreciated.

Leafhoppers
Leafhoppers have been evident on red, sugar, and Norway maples, redbud, and purple-leaved plum for approximately 3 weeks. Leafhopper should be sprayed when 12 insects per 4 branch sample are detected. Air blast sprayers are helpful in getting thorough coverage. Damage from leafhopper reduces winter hardiness of affected shoots, causes hopperburn, and causes stunting due to reduced internode space on developing growth. In addition, multiple leaders develop. For those growers with Pot-in-Pot production leaf appearance is critical as these trees will likely be sold in leaf, creating an added incentive to spray. For control try carbaryl, Diazinon, Dursban, Talstar, Tempo, Decathalon, Scimitar, and Tempo SC Ultra.

Source: Commercial Insect and Mite Control for Trees, Shrubs, and Flowers, U.T. Pub. PB1589, Revised Feb. 1999.
Johnson and Lyon. Insects that Feed on Trees and Shrubs.
1991. Cornell University Press.

 
Cultural:

Spacing Container Plants
Proper spacing is important to produce high quality plants. Adequate exposure to sunlight is necessary for trees to photosynthesize at the maximum rate, supporting the maximum growth possible. Leaves are shed if they do not produce more food than is required to maintain them. The leaves most prone are those in heavy shade on the lower trunk. Leaves and small stems along the stem are critical to developing caliper. Caliper development is crucial to developing a strong trunk that doesn't need to be staked and for growing a plant to the desired size. In addition, plants that are too close often look great as a block but when individual plants are pulled they have one-sided or sparse canopies. During production plants should not be closely spaced for any longer than is necessary. As a general guideline no more than 3 weeks to prevent canopy problems and less, especially if rainy or humid, to avoid the disease problems. Additional benefits are ease of spraying, pruning, and training leaders.

How much space is adequate? Check your plants several times throughout the day to see the shade pattern and adjust your space accordingly. Plants intended to grow in full sun (not under a shade structure) will do best with full sun or at least 6 hours of sunlight. During the hottest part of summer some midday shade can be beneficial but that is not the type of shade created when plants are too closely spaced. Consider that 15 gallon pot-in-pot socket pots are typically spaced on 5' centers and 25 gallons at 6' centers. .

Source: Spacing and Pruning Trees in Field Nurseries. UT Pub. PB 1458.


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