Pecan Diseases

From Bugwoodwiki

Diseases, especially pecan scab, can be a major limiting factor in pecan production. Disease losses can only be prevented with a carefully planned spray program. All fungicides currently available for pecan disease control must be used as protectants to prevent infection. Fungicides will not have much effect on infections that occur before treatment. Fungicides are applied on a regular basis to maintain a protective barrier over the fruit and foliage of the pecan tree. The protective barriers may be internal, as in the case of Orbit, or external, as with TPTH and other pecan fungicides. The standard disease prevention in Georgia calls for fungicide applications on a 14 day interval from bud break until pollination (three applications). This is the period of most active leaf growth. A 14 to 21 day interval is suggested from pollination until shell hardening. This schedule will need adjustment depending on season, varieties, etc. For most older groves, bud break should be considered as the stage when green tissue is readily visible on the dominant variety. Most growers modify the standard program to best fill their individual needs. Even though fungicides are applied on a rather tight calendar schedule, regular orchard scouting can aid a grower’s disease control program in four ways:

The most obvious benefit of regular orchard scouting is to let the grower know if his disease control program is giving satisfactory results.

The fungicides commonly used to control scab and most other diseases do not control powdery mildew or zonate leafspot. These diseases occur occasionally and require special control measures. Regular scouting can aid a grower in knowing if and when these measures are needed.

Pecan varieties vary greatly in their susceptibility to scab. Historically, the general trend has been for varieties regarded as scab resistant to become susceptible with the passage of time. Many growers spray varieties believed to be scab resistant much less than varieties known to be scab susceptible. Orchard scouting can inform a grower if the disease control program used on a variety believed to be scab resistant is adequate. “Stuart” is the leading pecan variety in Georgia. It has been considered more or less scab resistant for many years. There has, however, been a continuous increase in scab on Stuart. Some growers have suffered losses that could have been avoided with closer observation.

The importance of pathogen populations becoming resistant to fungicides has been clearly demonstrated with pecan scab. Regular scouting should provide early clues to problems of this nature.

• Pecan scab (Cladosporium caryigenum)
• Downy spot (Mycosphaerella caryigena)
• Powdery mildew (Microsphaera penicillata)
• Zonate leafspot (Cristulariella pyramidalis)
• Fungal Leaf Scorch
• Brown spot (Cercospora fusca)

--See also the archived article Pecan Diseases.