Win the Battle Against Powdery Mildew

Powdery mildew is one of the most easily recognizable diseases of landscape and garden plants.  While most fungal pathogens are favored by wet weather, powdery mildew pathogens favor high humidity but not wet conditions.  Shady areas, areas with poor air circulation, and low areas that trap damp air are ideal environments for disease development.

Powdery fungal growth is an easily recognizable symptom of powdery mildew disease.

A wide range of plants are susceptible to powdery mildew:  annuals, perennials, shrubs, ornamental and fruit trees, small fruit, and vegetables.  While symptoms appear similar on most plants, powdery mildew fungi are usually host-specific.  For example, powdery mildew of dogwood cannot infect pumpkins or cucurbits, and powdery mildew of cantaloupe will not infect phlox or rose. 

Typical symptoms of powdery mildew include dusty fungal growth on surfaces of leaves and on young succulent plant tissue (Photo 1 & 2).  Powdery mildew may occur as isolated blotches or cover entire leaves, stems, buds, or flowers.  Early infections can lead to leaf stunting, curling, or other deformation (Photo 2).  Infected fruit may become disfigured or fall prematurely (Photo 3).  In the fall, small fruiting structures that resemble pepper flakes may be visible to the naked eye (Photo 4 & 5).  These structures contain overwintering spores that serve as inoculum for the following year.

Infection of young growth may result in stunted or deformed tissue.

Management of powdery mildew begins with prevention.  Plants should be properly spaced and thinned for improved air circulation and more rapid leaf drying.  Resistant cultivars are available for crabapple, dogwood, phlox, zinnia, cucurbits, and several other plants and are an excellent means of disease prevention.

Fruit infections ultimately lead to yield loss.

Fungicides usually are not warranted when cultural practices are implemented.  Early season infection, however, may require one or more applications of effective fungicides.  Homeowners have a wide range of fungicides available.  However, they should carefully read labels to confirm that the selected fungicide may be used legally on select plants or that the fungicide is suitable for edible plants.

Fungicides effective against powdery mildew include:

·         Chlorothalonil – broad spectrum protectant; ornamentals, some fruit and many vegetables

o   Bonide Fungonil concentrate or Fungonil RTU

o   Ortho MAX Garden Disease Control

o   Sevin Daconil

·         Myclobuanil – systemic; ornamentals, most fruit and vegetables

o   Spectracide Immunox (only Immunox Multi-purpose Fungicide Concentrate is labeled for use on fruit and vegetables)

·         Propiconazole – systemic; ornamentals, some fruit

o   Bonide Fungonil RTS (different from other Fungonil products, which contain chlorothalonil)

o   Bonide Infuse

o   Fertilome Liquid Systemic

·         Tebuconazole – systemic; ornamentals only

o   Bayer Advanced Disease Control

o   Bonide Rx Systemic

·         Triflorine – systemic; ornamentals only

o   Ortho Rose Pride Disease Control

Late in the growing season, powdery mildew fungi produce small, black overwintering structures that can serve as inoculum the following year.

Why Topping Hurts Trees

Why Topping Hurts Trees from www.treesaregood.com

Topping is perhaps the most harmful tree pruning practice known. Yet, despite more than 25 years of literature and seminars explaining its harmful effects, topping remains a common practice.

What is Topping?

Topping is the indiscriminate cutting of tree branches to stubs or to lateral branches that are not large enough to assume the terminal role. Other names for topping include "heading," "tipping," "hat-racking," and "rounding over."

Topping is often used to reduce the size of a tree. A homeowner may feel that a tree has become too large for his or her property, or that tall trees may pose an unacceptable risk. Topping, however, is not a viable method of height reduction and certainly does not reduce future risk. In fact, topping will increase risk in the long term.

Topping Stresses Trees
Topping can remove 50 to 100 percent of a tree’s leaf-bearing crown. Leaves are the food factories of a tree. Removing them can temporarily starve a tree and trigger various survival mechanisms. Dormant buds are activated, forcing the rapid growth of multiple shoots below each cut. The tree needs to put out a new crop of leaves as soon as possible. If a tree does not have the stored energy reserves to do so, it will be seriously weakened and may die.

A stressed tree with large, open pruning wounds is more vulnerable to insect and disease infestations. The tree may lack sufficient energy to chemically defend the wounds against invasion, and some insects are actually attracted to the chemical signals trees release.

Topping Leads to Decay
Correct pruning cuts are made just beyond the branch collar at the point of attachment. The tree is biologically equipped to close such a wound, provided the tree is healthy enough and the wound is not too large. Cuts made along a limb between lateral branches create stubs with wounds that the tree may not be able to close. The exposed wood tissues begin to decay. Normally, a tree will "wall off," or compartmentalize, the decaying tissues, but few trees can defend the multiple severe wounds caused by topping. The decay organisms are given a free path to move down through the branches.

Topping Can Lead to Sunburn
Branches within a tree’s crown produce thousands of leaves to absorb sunlight. When the leaves are removed, the remaining branches and trunk are suddenly exposed to high levels of light and heat. The result may be sunburn of the tissues beneath the bark, which can lead to cankers, bark splitting, and death of some branches.

Topping Can Lead to Unacceptable Risk
The survival mechanism that causes a tree to produce multiple shoots below each topping cut comes at great expense to the tree. These shoots develop from buds near the surface of the old branches. Unlike normal branches that develop in a socket of overlapping wood tissues, these new shoots are anchored only in the outermost layers of the parent branches and are weakly attached.

The new shoots grow quickly, as much as 20 feet (6 m)in one year in some species. Unfortunately, the shoots are prone to breaking, especially during windy or icy conditions. While the original goal was to reduce risk by reducing height, risk of limb failure has now increased.