Originally created 08/11/00

Rot fungus winters in dead bark



Q: Can you tell me what causes brown spots on fruit such as apples and pears? What can you do to ensure that the fruit in the future will not like this?

A: Let me first address the apples. There are three rots that can affect apples. They are black, bitter and white rots. They vary in their appearance, so I'll describe each rot, what it looks like and how to control it.

Black rot infections develop very early in the growing season, during what we call silver tip (when the swollen buds first break and develop a silver tip). This infection causes a late-season rot that begins in the core and emerges at the calyx end as the fruit ripens. Secondary infections can occur anywhere on the fruit in late June, July and August. Black rot decay typically shows alternating rings of brown and black.

Black rot over-winters on the bark of dead wood in the trees and on the ground. From December to March, spores are produced and spread to fruit buds by rain from dead wood. Spores wash into the buds and remain there inactive until the buds start to swell at the silver-tip stage. The fungus is dormant until the fruit starts to ripen. Then it becomes active and rots the fruit.

Black rot control requires good sanitation. This means virtual elimination of all dead wood from the tree and on the ground. Make sure you also remove wood that falls to the ground after pruning.

Year-round attention to sanitation greatly reduces the amount of black rot fungus present in an orchard and may give adequate control. Spraying with a fungicide such as Captain one time at silver tip is usually enough for an entire season.

Bitter rot infections typically develop more than a month after petal fall. Lesions begin as small, sunken brown spots that grow up to 1 inch in diameter and sometimes are surrounded by a red halo. The halo is especially visible on green or yellow fruit. Concentric circles of pinhead-size black, round fruiting bodies often form within lesions. In wet weather, masses of cream, salmon or pink spores are produced. Bitter rot progresses to the core of the fruit in a V-shaped pattern. This differs from white rot, which forms a cylindrical rot pattern going to the core.

Like black rot, bitter rot survives the winter on dead bark and on mummified fruit, particularly those that hang on a tree. So dispose of all dead wood and any mummified fruit for control. Begin your spray program for bitter rot after petal fall every 14 days and continue up to six weeks before harvest.

White rot is a very serious late season rot. White rot lesions begin as small, circular, brown or tan spots. A red halo sometimes surrounds spots on green or yellow fruit. The halo may be purple to black on red apples. As the lesions enlarge, a cylindrical-shaped rot progresses to the core. This can be used to separate it from bitter rot, which forms the V-shaped rot I mentioned earlier.

Under warm conditions, which often prevail before harvest, white rot progresses rapidly. When the weather is warm, white lesions are brown to tan, watery and soft. Rot may consume entire fruits in a few days. Under cooler conditions, white rot is firm and darker brown color. During cooler harvest seasons, black rot and white rot lesions are difficult to separate.

The white rot fungus over-winters in the bark of dead wood on the ground and in the tree. Once again, sanitation is very important.

Spraying for white rot should begin six, four and two weeks before harvest.

Now, let's talk about pears. It sounds as though you may have pear scab. This is a fungal disease that over-winters in fallen leaves and twig blisters, releasing spores in the spring. The disease is most active in cool spring weather.

With scab, as the pears form, they develop olive-brown spots that turn corky and dark brown. Affected fruits may also crack and fall before they mature. There may be blisters or cankers on some of the twigs.

For control of scab, the sanitation is the same as with the apples. But also, prune any twigs that may have the above described blisters or cankers. You begin the spray program for this disease during bud break; again during the white bud stage; then two more times after the petals fall.

As I tell gardeners in our area, growing some fruits can be challenging. In many cases you need to follow a spray program if you are going to be successful. Fruit growers should get a fruit spray guide from any Extension office. That way they can have it to use as a reference tool, to know what to use and when to use it.

Many sprays you can buy at garden centers have a combination of insecticide and fungicide, so you can also help take care of any insects that may cause problems.

Sid Mullis is director of the University of Georgia Extension Service office for Richmond County. Call him at 821-2349, or send e-mail to smullis@uga.edu. The offices that serve Richmond and Columbia counties have a Web page at www.griffin.peachnet.edu/ga/columbia.