When should fungicide applications for control of pecan scab disease begin?

Randy S. Sanderlin  |  5/8/2012 11:08:08 PM

A frequently asked question about pecan disease control is, “When should the first fungicide application on my pecan orchard begin each year?” This is an important decision, and not always one with an easy answer. Following is a discussion that should help in making the yearly decision on when to begin the pecan fungicide application program.

The safest answer for someone who has responsibility for answering this question is to be conservative and suggest spraying early and spraying often. By doing that, even though the pecan grower may make some unnecessary sprays, the person making the recommendation looks good because loss from scab disease was prevented. In order for growers and those who prepare fungicide schedule recommendations to err on the side of good disease control, printed fungicide schedules usually have more applications listed than are really necessary for every orchard in every year.

To get the best use of pecan management money and obtain adequate control of scab disease, there are several factors that can be considered to help make a decision on the appropriate time to start your scab disease control program. These include the disease history of the orchard, cultivars in the orchard, rainfall forecasts and temperature. These factors are discussed separately, but of course, they need to be considered together when making the decision on when to start fungicide applications.

In March through mid-May, scab disease is essentially limited to new leaf and stem growth because the initiation of nut growth has not begun during that time. Infection of stems can limit terminal growth, and extreme infection can even kill terminals. Infection of leaves can reduce food production, and if leaves are killed the trees will waste energy in the growth of new leaves. Infection of leaves and stems reduces the overall growth of trees and reduces nut yield. Infection of leaves and stems also provides an abundance of scab pathogen spores adjacent to the growing nuts, making control of nut scab even more difficult.

Orchard History: Some orchards seem to develop damaging levels of scab disease on the leaves and stems almost every year. Orchards with a history of scab disease on leaves and stems are often located in a topography with poor air circulation, have persistent fog periods, and crowded tree spacing. In these situations, early season fungicide applications are needed to prevent leaf and stem disease. Unless infection is historically severe very early, it is usually not necessary to begin fungicide applications at true bud-break. Even in orchards that tend to develop leaf and stem infection, it is generally not needed to begin fungicide application until about an inch of leaf growth has developed throughout the trees of the most susceptible cultivar in the orchard.

Many orchards seldom have a significant level of early season scab on the leaves or stems. In such orchards it may not be vital to apply fungicide to protect against scab infection in the initial leaf growth period of the orchard. Or if one cultivar in the orchard does have a problem with leaf/stem scab it may be useful to target just that cultivar for an early application or two. In many instances it would be sufficient to begin fungicide applications in early to mid-May. The history of the orchard should provide useful insight into the best timing of the first fungicide spray.

Cultivar Susceptibility: Some cultivars in an orchard may have more of a problem with leaf and stem scab disease than others. Just as the orchard is composed of different pecan cultivars, there are different races in the scab pathogen population of an orchard. Some orchards may have a resident pathogen population with a higher level of virulence than other orchards composed of the same cultivars. The mixture of pathogen races and virulence will vary from orchard to orchard and is one of the factors that will determine the tendency of orchards to have or not have significant scab disease on the leaves and stems.

Rainfall: Perhaps the most important thing to know about the use of fungicide to control pecan scab disease is this. Scab disease control with fungicides is preventative, fungicide protects against infection. Fungicides do not reverse the effects of infection once the pathogen is established. Infection occurs during rainfall periods – thus fungicide should already be on trees during a rainfall episode to prevent infection from occurring. Most fungicides are believed to be effective for at least two weeks following application. Rainfall forecasts are crucial for scheduling fungicide applications to get adequate control without over-spraying. For example, if an orchard has an occasional problem with leaf/stem scab and a rainfall period is forecast, it would be wise to have a fungicide applied to protect against infection during the rainfall period. Conversely, if leaf/stem scab is not typically a problem in a specific orchard and weather forecasts indicate a low rain probability it will not be necessary to apply fungicide during this period.

Temperature: Temperature is not normally included as a factor for scheduling fungicide sprays, but it should be considered in the early spring. It is not unusual to have a warm period that initiates bud-break, followed by an extended cool period when growth is static. Experience has suggested that when daytime temperatures do not get above the 50’s or low 60’s little infection will occur even with the presence of rainfall. Fungicide protection during such cool periods is not necessary.

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