Plant Diseases

Plant diseases can reduce the productivity and beauty of our gardens and landscapes. Managing plant diseases does not begin with sprays. Understanding plant disease begins with understanding the disease triangle, which demonstrates the connection between the plant, the disease-causing organism, and the environment. If you can interrupt any of the three components, your plants have a better chance of thriving disease free.

The easiest place to start is to plant a resistant species or variety of plant. When you choose plants, look for those that list disease resistance traits. You should also become familiar with the varieties available and choose those that match your planting environment.

Next, you can avoid disease problems by avoiding the environmental conditions that promote disease development. Most disease-causing fungi and bacteria flourish when foliage remains wet for extended periods of time and when temperatures are within a certain range. To put it simply, the more often it rains or you turn on the sprinklers, the more the potential for disease development increases.

Finally, there is the third part of the triangle, the disease-causing organism itself. Sanitation means not bringing diseased plant material into the garden as well as removing diseased plant to reduce spread of the disease. Sprays to kill or hamper development of a fungus or bacteria are also a part of this third part of the triangle. When the other practices fail to prevent plant disease problems sprays may be necessary.

Best Diagnostic Resources

There are two excellent places to start when trying to determine the cause of injury.

The first is the city of Austin Watershed Protection Department. They have an excellent brochure entitled Diagnosis of Plant Problems to help you distinguish between disease and insect damage. You can download it as a PDF or pick up a copy at local nurseries or libraries.

The second is the Texas Plant Disease Handbook. It characterizes most diseases in these categories: Air Pollution Injury, Chemical Damage (Phototoxicity,) Drought Symptoms, Plants that Grow On Other Plants (like ball moss,) Viral Diseases, Wilt Diseases, Winter or Frost Injury, and Withering and Scorching of Foliage. They also list disease by name and types of plant. It includes color photos throughout.

You can use these resources to try to diagnose the situation, or to be better informed when you ask for help. Local nurseries, arborists, Travis County Master Gardeners, and the Texas Plant Disease Diagnostic Lab are all good places to go for advice and diagnosis.

Most Common Plant Diseases for Austin

Below is a list of the most common disease issues found in Austin.

Diseases on Multiple Ornamental and Food Crop Plants

Cotton Root Rot – Fungal organism, Phymatotrichum omnivorum, that can infect over 2,000 different plants. Most common symptoms are all leaves suddenly turning brown and plant dying quickly. Soil borne diseases that can affect one plant at a time or an entire area. Onset usually occurs once daytime temperatures are very warm.

Crown Gall – Bacterial organism, Agrobacterium tumefaciens, that infects trees and shrubs. Symptoms are the appearance of rounded nodules that can grow up to one foot in diameter. Most often found at soil level but sometimes appears at the crown. Disease restricts water movement through plant tissues and becomes noticeable if plant is stressed by freezing temperatures or drought.

Root Knot Nematodes – Root knot (nematode – Meloidogyne spp.) can attack any plant, although some are specific to the host. Above ground symptoms are wilting during periods of moisture stress, stunted plants, chlorotic or pale green leaves, and reduced yields. The real symptoms are actually happening underground. Infected roots well swell and form knots or galls, be retarded in growth, and lack fine feeder roots. Root rotting may develop late in the season.

Turfgrass and Lawn Diseases

Brown Patch – Circular patches up to several yards in diameter commonly occur in the fall through early spring when these grasses are approaching or emerging from dormancy or evening temperatures are below 68°F. Promoted by wet weather or frequent irrigation.

Take-All Root Rot – Yellowing of grass and darkening of grass roots symptoms appear in the hot days of summer. The roots can become so rotted that the grass can be easily pulled up. Excessive nitrogen seems to promote the fungus spread.

Ornamental Plants

Common Oak Diseases, including Oak Wilt

Twig blights and winter injury on Cedar, Juniper, Arborvitae, and Cypress

Rose Diseases

Rose rosette – All roses are susceptible but Knock Out Roses are particularly prone to the disease. A common symptom is “witches’ brooms,” which are brush-like clusters of shoots and branches that originate at the same point. Other symptoms may be more subtle (like thickening of canes and excess thorns.)

Fungal Leaf Spot – Characterized by roundish spots or irregular lesions on leaves, usually with a dry, brown or black raised center. Lesions are scattered at random. It is
unlikely to be fungal leaf spot if the damage is exclusively at the leaf margins or on the veins.

Fruit Tree Diseases

Fire Blight – bacterial organism – Erwinia amylovora that infects pears. Most recognizable symptom is the die back of branches starting at the tip that resembles fire damage. Symptoms occur early in the season.

Bacterial Canker – Pseudomonas syringae on peaches and plums. Cankers develop during the fall and winter but are not visible until late winter and early spring when the tree breaks dormancy. Damaged areas are slightly sunken with oozing sap that smells sour.

Vegetable Diseases

Tomato – Early Blight, a fungal organism – Alternaria solani. The first symptoms are small, black lesions mostly on the older foliage. Spots enlarge, and by the time they are one-fourth inch in diameter or larger, concentric rings in a bull’s eye pattern can be seen in the center of the diseased area.

These are just a few of the most common examples. There are many, many, other fungal and bacterial diseases that may be present.