Texas Peach Handbook

Chapter 10: Peach Diseases

Chapter 10 PeachDiseases 95 P each orchards are subject to numerous disease organisms that can decay fruit or cause defoliation or other serious injury to all or parts of trees. In our climate, even in drier parts of the state, organic production of peaches is unprofitable commercially. Diseases of foliage and fruit are generally driven by rainfall and humidity and are therefore more problematic in wetter parts of the state. This chapter is not intended to be an exhaustive discussion of disease biology and epidemiology but merely an overview of some of the more problematic diseases across the region. One in a series of important references, Compendium of Stone Fruit Diseases (edited by J. Ogawa, E. Zehr, G. Bird, D. Ritchie, K. Uriu, and J. Uyemoto and published by the American Phytopathological Society, 1995) is the single fullest reference for growers looking for more substantial detail on peach diseases. Because the registration of disease control products changes constantly, we have avoided discussion of specific products and rates. Instead this discussion centers on understanding the biology of the most common diseases and presenting cultural strategies that are important in disease management. Diseases of Foliage and Fruit Brown Rot The single most destructive disease affecting peach fruit around the world is brown rot,caused by the fungal pathogen Monolinia fructicola. During the spring, with sufficient inoculum and high rainfall, brown rot can infect and destroy blossoms and young vegetative shoots.Once fruit are set,they remain susceptible to infection through the duration of the ripening process. Infection on ripening fruit can be seen by a rapidly spreading brown lesion that eventually engulfs the entire fruit. When left in the tree, infected fruit eventually dehydrate and remain affixed to limbs.These“mummies”are a major source of inoculums for subsequent infections. Latent fruit infections can be highly problematic after harvest and can dramatically shorten the shelf life of fruit. In storage, a single infected fruit can rapidly deteriorate, produce spores, and give rise to infection of adjacent fruit in the same container or storage facility. Brown rot overwinters as mummies in trees or on the ground and in infected vegetative tissue. Removal of blighted shoots and mummified fruit is an integral part of overall disease control. Conidia, which give rise to spores for subsequent infection, can be formed on previously infected tissue at temperatures of 41°F and higher. In terms of the impact of weather on infection, it really does not matter how much it rains; the issue is how long the canopy of a tree remains wet. With as little as three to five hours of canopy wetness and temperatures of 65–70°F, significant infection can take place. Infection can occur at any temperature from 41° to 86°F and is most pronounced between 65° and 70°F.  Brown rot mummies can be retained in the tree canopy over the winter and give rise to disease outbreaks the following year. Care should be taken to remove mummies from the orchard during pruning.  Brown rot–infected fruit begin to dehydrate and show spore-producing structures that can lead to higher disease severity. 96 Chapter 10 Commercial management of brown rot is dependent on protective fungicides, which are typically applied several times during the growing season. The frequency of application depends on the amount of inoculum in the orchard and the frequency and duration of wetness in the canopy. Orchard sanitation can facilitate disease management by reducing inoculum, and canopy management through dormant and summer pruning can help reduce the duration of canopy wetness following precipitation. In our state, commercial organic production of peaches is at best very difficult. Homeowners looking to manage brown rot with organic materials such as sulfur will be dependent on sanitation, canopy management, and luck to ripen a crop. With frequent rainfall, there will be significant losses, but these techniques may prove acceptable in drier years. Neither author recommends commercial organic peach production in any area of the state. Under high rainfall seasons, management of brown rot can be challenging even with the frequent application of commercial fungicides. Peach Scab Thesecondmostimportantfungaldiseaseof fruitintheSouthispeach scab, caused by the fungal pathogen Cladosporium carpophilum. Scab is common on peaches and nectarines in warm, humid growing areas of the world and is sometimes found on plums. Leaves and shoots can become infected, but symptoms are more noticeable on fruit. Mild peach scab infection results in a spotted cosmetic blemish that...