Top Ten Diseases

Larry Sagers Horticultural Specialist Utah State University Extension Service Thanksgiving Point Office © All Rights Reserved

The top ten list. You hear them for everything. You hear celebrities expound them but have you ever heard one done by a plant pathologist. I am indebted to Dr Sherman Thomson Utah State University’s Extension plant pathologist, emeritus for this list..

“Powdery mildew is probably the most frequent disease cause by a microorganism in our state. It is very common on roses, pansies, columbines and many other ornamentals. It also attacks apples, squash and many edible crops. It is the exception to the general rule that fungi require abundant moisture to develop.”

“Prevent the disease by cultural practices such as wide spacing of the plants and using resistant cultivars. Pathologist should encourage plant selection and growing techniques that help prevent problems. Fungicides are useful but must be used frequently to prevent the disease.”

White mold is another fungus that attacks many overwatered flowers in late summer. Most bedding plants are susceptible but marigold and petunias are the most frequently damaged. The plants die and when the stems dry out. Prevent problems by plant rotation and careful watering.

Verticillium wilt is another serious problem on Thomson’s list. It is common on many ornamentals, potatoes, and tomatoes. The fungus is in the soil and plugs off the water conducting tissues. There are no chemical controls and the fungus lasts in the soil for up to twenty years.

One of the most preventable diseases is root rot. “We kill thousand of plants each year because we overwater them. Avoid planting susceptible plants in poorly drained soils. Reduce irrigation to satisfy evapotransporation be careful, improve the drainage and mulch judiciously. Fungicides are available but do little unless cultural practices are changed to prevent problems.” Aspen leaf spot is a common but not life-threatening pest. It defoliates even native trees. The disease starts in wet springs and infections continue until we get warm dry weather. Fungicides can be used but are not usually needed. Daconil is a good choice for homeowners.

Cytospera canker is a very widespread problem on woody plants. It often produces gum and other problems. The disease usually attacked trees that are already weakened or stressed. Keep the tree vigorous and prune out the damaged branches.

Viruses are proteins that must reproduce inside of living plant tissues. Some are serious and do significant damage while others have little affect on the production or the health of the plant. One example is Rose Mosaic virus. The plant still grows and blooms but it can never be cured. When a plant is infected with a virus it has it for life.

Iron chlorosis is a widespread problem. It causes necrotic leaf spot and other symptoms that are often misdiagnosed. If the plant needs iron and you treat it, the other problems go away. For severe problems in heavy alkaline soils, Thomson recommends Iron 138 or Millers Ferriplus Iron.

Mechanical damage to plants takes many forms. Careless operators of string trimmers or lawn mowers cause serious problems in the landscape each year. Poor quality pruning, careless children or pets and vehicle traffic all add to the problems.

Thomson’s final disease is chemical damage. “Herbicide misuse is an ever increasing problem because people do not follow the label directions. There are two kinds of damage. The first is the curled twisted leaves and stem injury caused by lawn weed killers containing 2,4D or dicamba. Do not use these near susceptible plants or when temperature get above 85 degrees. Triazine injury causes the death of many trees and shrubs. People put it out as a soil sterilant and the trees pick it up through their roots. Avoid using any of these products in or around home landscapes.

Garden Tips You can access additional information from Thomson on plant diseases in Utah at http://extension.usu.edu/coop/ag/environ/ipm/