At this time of year, a frequently asked question is, “When should I make the first fungicide application on my pecan orchard?” This is an important decision, and not always one with an easy answer. The safest answer for someone who has some 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 schedule writers to be better safe than sorry, printed fungicide schedules usually have more applications listed than are really necessary for every orchard in every year. If a pecan grower wants to make the best use of their pecan management money and get adequate control of scab disease, several factors can be considered to help decide on the appropriate time to start a 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 all need to be considered together when making the decision on when to start fungicide applications.
Scab disease in March through mid-May essentially is limited to new leaf and stem growth because the initiation of nut growth has not yet begun. 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, then the trees will waste energy in the growth of new leaves on the terminals. Infection of leaves and stems reduces the overall growth of trees and reduces both quantity and quality of nut production. 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 challenging. The conventional logic is that the greater the infection of leaves/stems, the more difficult it is to control nut infection. All of us have witnessed severe nut scab disease when there was no infection of the leaves and stems.
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 topography with poor air circulation, persistent fog periods and crowded tree spacing. In these situations, obviously early-season fungicide is needed to reduce the level of early-season infection. 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 some leaf and stem infection, I generally do not recommend making a fungicide spray until there is enough foliage growth to catch some of the fungicide. There should be about an inch of growth 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 stages 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, I suspect it would be sufficient to begin fungicide applications in mid-May. The history of the orchard should provide useful insight into the best timing of the first fungicide spray.
Cultivar Susceptibility: As mentioned in the previous section, 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 of the scab pathogen population in the 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. Infection occurs during rainfall periods – thus fungicide has to be applied before a rainfall. 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 frequent rainfall period is forecast, it would be wise to have a fungicide applied to protect against infection during the rainfall period.
Temperature: We don’t normally include temperature 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 with the initiation of bud-break, followed by an extended cool period when growth is static. Experience has suggested that when daytime temperatures do not get above the 50s or low 60s, little infection will occur even with the presence of rainfall. Fungicide protection during such cool periods is probably not necessary.
Below is a table of the currently recommended fungicides for pecan disease control. One new product has been added since last year. The fungicide Quash made by Valent is another SBI type fungicide in the same FRAC Group 3 as Enable, Propimax and Bumper. In tests at the Pecan Station it was comparable in control to the other fungicides; it should be used at its maximum labeled rate of 4 ounces per acre. As always, to try to delay pathogen resistance to the fungicide groups and subsequent lost of disease control, please be aware of the various products in each FRAC group and avoid making repeated successive applications of products in the same group.
Pecan Fungicides List By Activity Group*
|
FRAC GROUP |
PRODUCT |
RATE / ACRE |
|
Group 30
Organotin |
Agri-Tin
Super-Tin |
7.5 oz
7.5 oz |
|
Group 3
DMIs |
Enable
Propimax
Bumper
Quash |
8 fl oz
6-8 fl oz
6-8 fl oz
4 oz |
|
Group M
Guanidine Acetate
(Dodine) |
Syllit
Elast |
2 lbs
51 fl oz |
|
Group 11
Strobilurin |
Abound
Sovran
Headline |
9.5 fl oz
3.2 oz
7.0 fl oz |
|
Group 3 & 30
DMI + Organotin |
Orbit/Super Tin
Enable/Agri-Tin |
4 oz & 3.75 oz
1.3 oz & 3.74 oz |
|
Group 6: Mix of
Groups 3 & 11 |
Stratego
Quilt |
10 fl oz
14 fl oz |
*To obtain the best control and reduce the chances of resistance to fungicides, use a rotation of fungicides from different FRAC Activity Groups, or a mixture of fungicides from different groups. The Fungicide Resistance Action Committee (FRAC) groups fungicides by the method of activity against fungi. Development of resistance to one fungicide in a group generally means a pathogen will be resistant to all of the fungicides in that group.