Pest Management in Peaches

In this report reduced fungicide/bactericide spray strategies are discussed for bacterial spot, peach scab and brown rot. Other diseases exist but most of them are usually controlled by the regular spray programs with the exception of oak root rot.

Bacterial spot. This disease is caused by the bacterium Xanthomonas arboricola. It is very unpredictable, meaning it occurs only in some years. Research at NC State University led by Dr. Dave Ritchie is underway to try to predict years of high disease pressure. For now, it seems that infection of fruit is most severe in years when frequent periods of rainfall occur 3 to 4 weeks following petal-fall (PF). The current strategy for controlling bacterial spot is to reduce inoculum levels using copper starting at late dormant with 2 lb (a.i.) of metallic copper. Copper applications are continued up to (PF), but rates are significantly reduced to avoid phytotoxicity. After PF growers spray Mycoshield (oxytetracycline) in weekly intervals or rotate Mycoshield with copper applied at low rates throughout the season. Despite this very aggressive spray program, chemical sprays are not totally effective on highly susceptible cultivars in years when conditions for bacterial spot are very favorable. Timing of spray applications is critical, meaning that applications are most effective if applied prior to a rain but with enough time for drying. Clemson University scientists Dr. Dale Linvill and Dr. Walker Miller have developed a computer model to predict infection periods based on weather data. The model was tested three years in a row (99-02) in grower orchards from SC and GA under supervision of Dr. Phil Brannen, UGA. Based on his experience, the model seems to have merit especially early in the season but may need some adjustments in regard to forecasts later in the season.

Peach scab. You can count on this disease every year. The fungus (Cladosporium carpophilum) causes spots on developing fruit making the fruit unmarketable. The fungus begins producing conidia about two weeks before shuck split (SS) and continues to do so until 4 to 5 weeks after SS. Protection of the fruit tissue with fungicides is critical during this period. Several fungicides are registered for scab control, including Bravo, Abound, Captan, and Sulfur. The compounds differ in their efficacy against scab and market price. The grower has the choice to primarily use the cheaper but less effective Sulfur and risk some scab development in a high scab pressure season or the more expensive products with higher efficacy such as Bravo or Abound. Many growers know that the early scab sprays are the most important ones and usually do not hesitate to use Bravo or Abound at that time followed by the less expensive Sulfur in the cover sprays. This program is usually effective against scab if disease pressure is not too high and it is relatively inexpensive. Although scab pressure ceases four weeks after SS, many growers keep adding Sulfur to their spray tanks for “extra insurance”.

Brown rot. Brown rot of peach is caused by the fungus Monilinia fructicola and is probably the major pre and postharvest disease concern. When environmental conditions are conducive, the disease can spread quickly and may cause significant losses. Estimates of disease losses during an epidemic in Georgia for 2001 were $4.3 million in direct losses and $ 1.5 million in fungicide costs. The disease is particularly dangerous if blossom blight is allowed to produce high inoculum levels and if rainfall is prevalent during the growing season up to harvest. Other inoculum sources such as old cankers, peduncles, mummified fruit and green fruit rot may produce sufficient inoculum for epidemic levels of brown rot in any wet year. The prolific production of conidia, which are spread through wind and rain, allows for rapid epidemic development within an orchard or a region. Our research has shown that the demethylation inhibitor (DMI) fungicides such as Indar, Orbit, and Elite are still highly effective in controlling the disease although they have been used for almost three decades on a regular basis. Some DMI fungicides perform slightly better than others primarily due to the ability of the fungicides to bind to the fungal target enzyme. A shift of some M. fructicola populations towards reduced sensitivities has been documented, but recent research shows that this shift has not increased during the last six years.

Oak root rot. Prior to the widespread cultivation of cotton in many parts of the southeastern United States, the land was largely covered with hardwood forests. In this forested land, the oak root rot (ORR) fungus thrived and spread throughout the region. Today, the fungus can be found in the soil of most of the prominent peach growing areas - if not everywhere - should you just dig deep enough to find it. The ORR fungus can survive for up to a hundred years in root pieces deep down in the soil where degradation of organic material progresses very slowly. Many growers have a misconception that growing trees on ‘virgin’ peach land (i.e., land where peaches have never been grown before) is a solution to the problem of soil-borne pathogens that kill peach trees. Depending on the depth where the ORR is present and the amount of inoculum that exists in the soil, trees on virgin land can die from ORR well before they live out their productive lifespan. There is currently no rootstock commercially available that is resistant to ORR. Although GuardianTM rootstock is very helpful on sites with Peach Tree Short Life (PTSL), it is also very susceptible to ORR. Once living peach tree roots reach the infected root pieces remaining in the soil, they become infected. The fungus then migrates up the root system to the crown of the tree, which it essentially girdles and kills. The tree subsequently dies. On existing peach sites where trees have died from ORR previously, it is highly likely that the inoculum level in the soil is greater and closer to the soil surface than in ‘virgin’ land. Thus, one would expect trees on replant sites to die even sooner (Figure 1). We have actually excavated dying trees as young as in their second and third leaf where ORR was the cause of death on replant sites! MORE on oak root rot (pdf 919 KB)

There is basically nothing the grower can do to rescue infected trees. The only control option is to prevent the spread of the disease by pulling out adjacent, healthy trees. Even this method has its limitations and there is a cost in terms of lost production. The fungus, once established, will infect neighboring trees by root to root contact. The earlier the grower recognizes the existence of this disease in the orchard, the less damaging and more successful will be your option of prevention provided there is not a tremendous amount of inoculum at the site and only a few isolated trees are infected. We have several projects underway currently to address the growing problem of ORR to peach production in the southeastern U.S. These include the use of biological control against ORR, the potential development of a resistant rootstock through genetic engineering, and future work involving various cultural practices.