Powdery mildew
While you are confirming orders for 2005 liners, consider the impact of selecting resistant cultivars. Powdery mildew has been shown to severely reduce dogwood growth and flower bud number on flowering dogwoods (see Nursery Update #13). UT researchers found that C. florida 'Fragrant Cloud' and 'Cherokee Brave' were moderately resistant to powdery mildew. 'Fragrant Cloud' was susceptible to powdery mildew in NC and AL, whereas 'Cherokee Brave' has shown more consistent resistance. Overall, Cornus kousa hybrids were resistant to powdery mildew, however, Constellation®, Ruth Ellen®, and Star Dust® developed powdery mildew on more than 70% of their foliage.

One way to plan for powdery mildew control next year is to assess the spore load on trees in your field this winter and on liners you receive in the spring. Powdery mildew overwinters as a small, dark ascocarp, and can be found by using a hand lens to scout buds and small twigs. This ascocarp on buds and leaf debris is the primary source of inoculum for spring infections. The fungus can remain in the bud over the winter and not produce ascocarps until bud swell in the spring, so continue scouting into spring. By scouting, growers can determine when/if the acsocarps are present and can use this information to prioritize spray applications. Well timed fungicide applications reduce the spore load by reducing infection from the overwintering spores and the subsequent secondary infections (up to 40 per year).

Eagle 20EW is a protectant and curative systemic fungicide labeled for dogwood powdery mildew. Use this summer on C. florida 'Cherokee Princess' resulted in drastic growth regulator effects. For nursery ornamentals the label states 6-12 fluid ounces per 100 gallons. Using the 10 ounce rate two times, 14 days apart, resulted in leaf distortion. Powdery mildew control was excellent.

USDA researchers in the Northwest are predicting reduced incidence of powdery mildew in container plants as growers move from overhead irrigation to water conserving micro-irrigation. Powdery mildew requires high humidity rather than wet foliage for spore germination.

Sources: Mmbaga and Sauve. 2004. Journal of Arboriculture. 30(2):101-106; Ranney, et al. 1994. Proceedings of the SNA Research Conference. 39:212-216; and Schmidtz, J. 2004 (September). The Digger.

Above: Powdery mildew on Cornus florida


Above: Ascocarps on leaf tissue
(Above image from UW Master Gardener training materials.)


Above: Distorted growth after fungicide application

Cultural:

Plant Spacing
When water and nutrients are abundant, light intercepted by the plant is the next possible limiting factor. While field production spacing is semi-permanent, (although some nurseries dig and transplant during production to enhance root systems), pot-in-pot spacing is very permanent. Getting spacing right is important in order to balance the need to maximize production on valuable land with plants receiving optimal light.

Research done by Clemson University factored in leaf area, leaf shape, leaf size, and light absorption characteristics with crown structure, shape, density, branch arrangement and site characteristics. Their model showed that the optimal spacing for 2-3 year old seedling red maples in 15 gallon containers is 4' 1". The nursery owner had perfected red maple spacing over the course of the past 10 years through trial and error and found 4' to be the optimum!

The research also points out that if the grower over-estimates space requirements at 4'6", then per acre the grower will underplant by 571 trees. At a very conservative wholesale price of $30 per tree the grower would lose $17,130. If the optimal is 3' 6" then the loss in production potential increases to $42,147 (assuming there is a market for the greater number of plants).

Keep in mind that the 4'1" won't hold true for all red maples. However, this model can be used for red maple cultivars and other plants, just by entering the parameters for that plant.

Source: Spacing: the final frontier. NMPRO, July 2004.