In A Nutshell - First Quarter 2010

Katherine S. Burnham, Sanderlin, Randy S., Hall, Michael J., Graham, Charles J.  |  5/20/2011 6:48:17 PM

Planning Your Scab Disease Control Program

February 24, 2010 First Quarter Newsletter

Planning Your Scab Disease Control Program
Randy S. Sanderlin

It may be difficult to think about pecan scab disease during the cold of winter; however, winter is a good time to plan your scab disease control program for the next season. Even though you cannot know when fungicide applications will be needed during the next season, now is a good time to plan which products to use and the sequence of application to reduce the chances of crop loss because of scab pathogen resistance to fungicides.

All fungicides are classified into groups based on the mode of action they use to affect fungi. The groups are designated by numbers or letters by the Fungicide Resistance Action Committee (FRAC). Fungicide products within a FRAC group all have the same basic biochemical activity against fungi. When a pathogen becomes resistant to one of the products in a group, it becomes resistant to all of the products in the group. These groups vary in the tendency of fungi to develop resistance to them. As a rule of thumb, it may be best to use products in the FRAC groups that have the greatest probability for pathogen resistance occurrence during periods when the pathogen population is relatively low. Usually this is early in the year before disease has increased, but it can also occur at other times when there is little scab disease present. The products in the FRAC groups with lower tendency for pathogen resistance can then be saved for later in the year when scab disease is usually more prevalent. The basic resistance management strategy is to avoid using a particular FRAC group any more often than necessary during the season, while still maintaining scab disease control. With products labeled for pecan use in four distinct effective activity groups, it should be possible in most years to not need to apply each group more than twice in a season. With rotation of fungicides, it should be possible to never have to use the same FRAC group in consecutive applications. At the Pecan Station, the first two applications are generally made with DMI and QOI group products. To avoid confusion when planning your fungicide program each year, it may be best to use just one product within a group. Of course, the same products within groups do not have to be used each year. When applied before infection occurs, with good tree coverage and at the recommended rate per acre, the products in the list below should provide effective protection against scab disease.

Most of the products included in the table below have been tested at the Pecan Station and have performed well for scab. Those products not written in BOLD letters have not been tested at the Pecan Station, and I have not seen any data on how they work against pecan scab. Often products can be labeled for tree nuts and not have a specific test on all trees included on the label. Products that have performed well in tests at the Pecan Station include Agri-Tin, Super Tin, Enable, Orbit, Propimax, Bumper, Orius, Elast, Abound, Sovran and the mixture products. Even though the mixed products usually work well for scab control, I have always had concern about the use of these products with regard to the potential for pathogen resistance development. These combination products contain reduced amounts of two active ingredients compared with the products containing only one of the active ingredients. The FRAC recommendation has always been that when active ingredients are used in combinations, the ingredients should each be used at the rate comparable to the concentration applied when each active ingredient is used alone. There are certain factors that will influence the selection of specific products in each FRAC group such as effectiveness, availability, price and convenience of use for your particular sprayer/orchard setup.

There are also several factors that determine when the first fungicide application is needed each year. These include scab disease history of the orchard, rainfall and temperature. Some orchards have a problem with scab disease on leaves and stems every year, while others seldom have significant early season infection. Factors that increase the tendency for infection of leaves and stems are poor air movement in the orchard, overcrowded trees and cultivar susceptibility. Rainfall or dense fog is the trigger needed for the scab fungus to initiate infection. Without a wetness period of several hours, there will be no new infections. Of course, the number one rule with regard to fungicide applications is that to prevent infection, fungicides should be applied to trees before the rainfall period associated with an infection cycle. Each fungicide application should protect susceptible tissue for at least two weeks after application. When temperatures do not get above the low 60s, there will not be much increase in scab infection even with rainfall. This situation often occurs during the early spring growth of leaves and can allow for the delay of the initial fungicide application or an increase in interval between the first and second application.

Using what you know about the scab disease history of your orchard, carefully monitoring weather forecasts and the leaf growth on your trees should help in making the best decision possible about when to make the first fungicide application of the year. While the trees are dormant is the best time to plan your fungicide use for the coming season.

Pecan Fungicides List By Activity Group*

FRAC GROUP

RESISTANCE RISK

PRODUCT

RATE / ACRE

Group 30

Organotin

Low

Agri-Tin 80WP

Agri-Tin 4L

Super-Tin 80WP

Super-Tin 4L

7.5 oz

12 fl oz

7.5 oz

12 fl oz

Group 3 DMIs

Moderate to High

Enable 2F

Orbit 3.6F

Propimax 3.6F

Bumper 3.6

Quash 50WG

Folicur 3.6F

Orius 3.6F

8 fl oz

6-8 fl oz

6-8 fl oz

6-8 fl oz

4 oz

6-8 fl oz

6-8 fl oz

Group M7

Guanidine Acetate (Dodine)

Moderate

Elast 400F

51 fl oz

Group 11 Strobilurin

High

Abound 2.08F Heritage 50WP

Sovran 50WG Headline 2.09F

Gem 500SC

9.5 fl oz

6.4 oz

3.2 oz

7.0 fl oz

3.8 fl oz

Group 3 & 30 DMI + Organotin

Same As 3 & 30

Enable/Agri-Tin

1.3 oz & 3.74 oz

Group 6: Mixture of Groups 3 & 11

Same As 3 and 11

Stratego 1.04F

Quilt 1.04/0.62F Adament 50WG

10 fl oz

14 fl oz

8 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.

Some Don’t Like It Hot: A Simple Hot-water Procedure to

Some Don’t Like It Hot: A Simple Hot-water Procedure to Reduce
Graft-transmission of the Pecan Bacterial Leaf Scorch Pathogen
Randy S. Sanderlin

The bacterium that causes pecan bacterial leaf scorch (PBLS) disease can be carried in the dormant scion wood collected from diseased trees. As the scion begins to grow, the bacterium also grows and moves into the newly developing tree. Infection can reduce the growth rate of young trees, which delays nut production and reduces yield throughout the life of trees. There are currently no management methods for infected trees, so it is worthwhile to try to prevent infection, especially of young trees.

The obvious way to avoid the pathogen passing through the graft wood is to collect graft wood only from trees without PBLS. Trees with the disease can usually be identified from mid-June through the fall by observing the foliage for symptoms. Foliage of infected limbs will develop a tan to brown discoloration beginning at the margin leaflets. This tan discoloration will progress in a smooth, unbroken pattern toward the center and base of leaflets. Affected leaflets soon drop from the leaf stems, and more leaflets become symptomatic. Typically, leaflet stems will be in various stages of leaflet loss, and the leaf stems will also drop as defoliation advances. Symptoms can vary in intensity during the summer and fall, so it may be wise to observe trees intended for graft wood collection more than once during this period to be confident there are no limbs with symptoms. Photos of PBLS symptoms are available on the LSU AgCenter Pecan Station website and other internet sites, http://www.lsuagcenter.com/en/our_offices/research_stations/Pecan/.

It is not always possible to check every potential source of graft wood for PBLS, and if you are buying graft wood, you usually have no knowledge of the status of the trees from which it came. Even if a tree is known to have PBLS, it may be the only source of graft wood of a particular cultivar. There is a simple inexpensive method of eliminating the bacterial pathogen from scion wood that should be used when it is not certain whether the wood was collected from non-infected trees. The procedure requires one additional step in the grafting process. Graft wood is collected and stored using standard methods. When the wood is removed from refrigeration to use for grafting, it can be given a hot-water treatment that kills the bacterial pathogen. The procedure consists of the following: (1) submerge graft wood in water at 115 to 125 degrees Fahrenheit for 30 minutes, (2) at 30 minutes remove from hot water and dip in room-temperature water for a few seconds to remove residual heat. The wood can now be used for grafting.

There are some points about the hot-water procedure that need to be emphasized. The wood should be submerged in water and not floated; a weight can be added to graft wood bundles to accomplish this. Several cultivars were tested for sensitivity to hot water, and although there is variation between cultivars, the ones tested (Cape Fear, Desirable, Schley and Pointe Coupee) could withstand 125°F for several minutes without any visible damage to the wood. However, they could not withstand 125ºF for the full 30 minutes without damage. Graft wood damaged by hot water will become brown just beneath the bark or have brown streaks in the wood. The wood should not be kept at 125°F throughout the procedure, and 125ºF is the maximum temperature that it should be allowed to stay at for a few minutes. In tests, a temperature of 115°F for 30 minutes was sufficient to kill the bacterium, so a temperature range of 115 to 125°F allows some flexibility in carrying out the procedure and reduces the level of preciseness needed. An easy way to conduct the procedure is to heat water to 125°F, allow the temperature to stabilize then remove the heat source, add the graft wood and start timing. Monitor the temperature with a thermometer and if it appears that the temperature will drop below 115°F during the half-hour period, reapply heat to the water. It was easy to do this using a large pot of water heated on a kitchen stove. When water was heated to 125°F in a Crock Pot, the temperature stayed in the right range for about an hour after the heat was turned off, and the lid was placed on the pot. The larger the volume of water heated, the slower it temperature of the water will change relative to the air temperature. If there is a need to treat 1000s of pieces of graft wood, such as with a commercial pecan nursery, then some type of container larger than a pot on a stove will be needed. An Extension Specialist with the University of Maryland working with a way to kill insect pests on cuttings with hot water designed a hot-water treatment unit using a standard stock water tank as the water reservoir and an instantaneous hot water heater of the type that can be purchased at Home Depot or Lowe's as the heat source. The basic design for this unit is discussed in an article published in the American Nurseryman in 2007, "Keeping the Heat on Pests: Using a hot water immersion system effectively can control certain plant pests during propagation." This article can be found on the internet by typing the title into a search engine.

It should be possible to greatly reduce graft transmission of the PBLS bacterium through scion wood by careful selection of non-infected trees for scion wood collection and use of the simple hot-water procedure just prior to grafting.

Although the hot-water treatment does nothing to prevent subsequent introduction of the pathogen into trees by insect transmission, it gives young trees a good chance to have a few years of growth without the early growth-stunting effect of the bacterium and without needing to remove infected trees in their first year of growth to reduce spread of the pathogen to other trees in a new planting. One cautionary note, the potential for pesticides getting into the water from graft wood pieces that had been sprayed the previous year was not tested. As a precaution, it would be best to use a dedicated container for the hot-water process and not your favorite gumbo pot. After all, filé and insecticide do not belong in the same pot.

Pecan Phylloxera

Pecan Phylloxera   
Michael J. Hall
email:

Beginning in April, galls or knots begin to appear on the leaf veins, leaf rachises, current-season shoot growth, catkins and nuts of pecan. These galls or knots are caused by small, "aphid-like" insects known as pecan phylloxera, Phylloxera devastatrix Pergande.

Severe infestations of pecan phylloxera have been reported to cause reductions in nut quality and quantity, premature defoliation and dieback of the current season's shoot growth. In addition to causing damage to the vegetative and fruiting structures of the tree, the galls also serve as host for a multitude of other insects, including the hickory shuckworm, Cydia caryana Fitch.

Pecan phylloxera are found throughout the indigenous range of pecan and the pecan-producing regions of the southeastern United States. In addition to pecan, pecan phylloxera has been reported to occur on other species of hickory. Pecan phylloxera can be a problem on native trees; however, it is a more serious problem in commercial orchards that are planted with large numbers of cultivars susceptible to this insect. Cultivars known to be highly susceptible to pecan phylloxera in Louisiana include Stuart, Success, Schley, Desirable, Caspiana and Cape Fear.

In Louisiana, pecan phylloxera are active from mid-March through early June. Three generations occur per year, with only the first generation causing damage. It should also be noted that pecan phylloxera infestations are cyclic, with periods of heavy infestations followed by extended periods of light to no infestations.

The life cycle of pecan phylloxera is quite interesting. They overwinter as an egg within the body of a dead female. Before they die, the females seek out a protected space somewhere on the tree to overwinter. These egg-containing females can be found under the rough bark of the larger limbs, within old galls or in any area of the tree offering protection. They are even found under the shells (carapaces) of dead scale insects.

Upon hatching from the eggs, the nymphs or stem mothers move onto the opening buds and begin to feed. As the stem mothers feed and develop, a gall begins to form around them, eventually enclosing them. Each stem mother creates an individual gall, but when infestations are severe, large masses are often formed, making it next to impossible to distinguish individual galls. Gall size varies from 0.125 to 0.625 inches in diameter. In addition to protection of the stem mother and her brood, the gall is also thought to serve as a "nutrient sink," providing an optimal source of food for the insects enclosed within.

The eggs deposited by the stem mother within the gall hatch and develop into winged (alate) females, sometimes referred to as winged migrants. The winged migrants are yellowish in color with smoky-black wings and emerge when the galls begin to split open in late April. The winged migrants can sometimes be confused with aphids that may also be found on pecan foliage at this time; however, the aphids do not have the uniformly dark-colored wings and dark legs. The winged migrants do not cause any damage, but they are responsible for the dispersion of the insect throughout the orchard.

Shortly after emergence, the winged migrants begin depositing eggs on the leaves. When large numbers of eggs are deposited, the leaves take on a yellowish tinge. The eggs deposited by the winged migrants hatch into wingless (apterous) males and females. Upon hatching, the sexually mature males and females mate, and an egg forms in the body of the female who, before dying, seeks out a protected area on the tree to overwinter.

Insecticides are the most common and effective way to control infestations of pecan phylloxera. It is very important to understand that when insecticides are used they must be applied before the galls form. Once the insect is enclosed within the gall, control is no longer possible.

In order to determine if an insecticide application is necessary, the presence or absence of pecan phylloxera must be determined. Pecan phylloxera can be observed with a hand lens or magnifying glass. Beginning at bud break, monitor trees regularly through mid-April, or later depending on geographic location. Particular care should be given to those trees infested during the previous year. If pecan phylloxera are not present, or even present in low numbers, then no insecticide application is necessary. If pecan phylloxera are present and the infestation appears heavy, then an insecticide application should be considered. When using insecticides be sure to use the correct one and apply it at the correct rate. To minimize the chance for pecan phylloxera to become resistant to an insecticide, do not use an insecticide with the same chemistry year after year. Be sure to rotate insecticides. Information on labeled insecticides and proper insecticide use can be obtained from the local extension agent or crop consultant. Unless all of the trees within an orchard are infested, insecticide applications should be limited to only those trees known to be infested. This will reduce insecticide costs and help to conserve beneficial insects within the orchard. If timed correctly and with proper spray coverage, a single insecticide application will normally control or, at the least, significantly reduce the number of pecan phylloxera in an orchard.

Finally, before applying any insecticide, be sure to check the pH of the water to be used for spraying. The pH of the water used for spraying should range from 5.5 to 6.5. The pH should be adjusted before the insecticide is added and should be checked again prior to spraying. Having the water adjusted to the proper pH will provide better knockdown and control.

Suggested Insecticides for Control of Pecan Phylloxera (rates expressed in amount of product/acre):

Lorsban 4E at 1.5-2.0 pt./acre

Provado 1.6F at 3.5-7.0 fl. oz./acre

Warrior at 2.56-5.12 fl. oz./acre

Centric 40WG at 2.0-2.5 fl. oz./acre.

Trimax Pro at 2.6 fl. oz./acre

image of In A Nutshell Newsletter
Rate This Article:

Have a question or comment about the information on this page?

Innovate . Educate . Improve Lives

The LSU AgCenter and the LSU College of Agriculture

Top