Almond Anthracnose - An ugly spring disease

Anthracnose (Colletotrichum gloeosporioides) is a damaging fungal disease that attacks the blossoms, leaves, fruit, and limbs of an almond tree. Infected leaves tend to develop water-soaked lesions that eventually fade in color (Figure 1). Defoliation can occur, but leaves often remain attached to the branches. Infected nuts often have a crater-like lesion in which the affected area turns a reddish orange(Figure 2). Often, the fruit may gum profusely as the fungus is able to penetrate into the kernel, killing the embryo. Affected nuts often remained attached to the spur(Figure 3). Shoots and spurs that bear infected nuts often become infected and die(Figure 4). Lack of culturing the fungus from these branches suggests that the wood does not die by direct fungal colonization, but by a produced toxin. Thus, this disease affects the current year’s crop as well as affecting the future crop by damaging and weakening the affected scaffolds.Figure 1: Almond leaves infected by Anthracnose.

Figure 2: Anthracnose infection of an almond. Note the profuse gumming coming from the kernal.

Anthracnose is most severe in wet, warm springs (> 60F). Orchards that have a history of anthracnose should be treated during bloom (pink tip forward) to help protect blossoms. Repeated fungicide applications may be necessary through the spring as trees should be protected before rain events. Fungicides provide protection for up to 7-10 days, and rain events occurring beyond this “control window” may require another spray. All varieties are susceptible to Anthracnose, but they vary in their degree of susceptibility. Butte, Thompson, Merced, Price, Peerless, Winters, Monterey and Fritz are the most susceptible varieties. Harvey, Carmel, Ne Plus Ultra, Padre and Mission are moderately susceptible, with Nonpareil being the least susceptible. Figure 3: Kernals killed by anthracnose often remain on the tree as dried, shriveled mummies.

Figure 4: Anthracnose infection of almond branches.

There are several fungicides that provide excellent control of anthracnose. These include the fungicide classes of Strobilurin/Boscalid (i.e. Pristine), DMI’s (Orbit, Quash, Indar, Inspire, etc.), strobilurins (i.e.Abound), and products that combine two different modes of action (Distinguish, Inspire, etc.). It is important to note that each class of fungicides should not be used in succession as rotating will help reduce the chance of resistance. If 3 or more fungicide applications are needed due to spring rains, a broad spectrum fungicide (Captan, Maneb, Chlorothalonil, or Ziram) should be rotated in to help provide control and decrease the chance of resistance. Please check the label to ensure an adequate post harvest interval.

Anthracnose has not been a problem over the past few years. This is probably due to the lack of winter and spring rains as well as the number of fungicides active on anthracnose. Often, fungicides applied for brown rot or scab control provide anthracnose control as well. Forecasted wet weather suggests that this disease may be more severe this spring.

UPDATE: DMI Fungicides and Botrytis Control

Last week, I suggested the use of DMIs (Quash, Indar, Inspire, Rally, etc.) for brown rot control spray program. A fellow reader shared some experiences with me regarding the use of DMI fungicides during the late bloom period. Although DMIs are good for brown rot, anthracnose, and have some activity against scab, shothole and rust, they do not provide control for the fungus Botrytis, which causes jacket rot and green fruit rot (Photo above). Botrytis is found everywhere and grows rapidly in cool, moist weather. It typically infects young almond jacket and fruit, causing infection and nut abortion. It is greyish in appearance, and affects varieties that cluster - Avalon, for example. It is not found frequently, but when found, is typically in orchards that have only used DMI fungicides. Therefore, if your last fungicide application is a DMI, I would advise to include a broad spectrum as a tank mix (chlorothalonil, Captan, Maneb, Rovral) to help reduce the incidence of Botrytis infection.

Any other thoughts on this?

Website about the biology of honey bees

Found this website about Honey Bee Biology based out of the Department of Entomology at UC Davis. Appears to be a "one stop shop" for information regarding bee biology, native bees, bee keeping, and more. Check it out!

Checking Thermometers to Monitor Frost Temperatures

By: Maxwell Norton (UCCE Merced)

During frost season it is important to precisely know how cold it gets in various parts of your orchard or vineyard. Affordable minimum recording thermometers are available through farm stores and are usually pretty accurate but you need to check them yourself every year because they can “drift” up or down over time. The “U” shaped minimum-maximum recording thermometers are usually accurate at one end or the other but rarely both. Don’t use a “freebee” or a decorative thermometer.

Fill a plastic bucket up with ice. Add enough cold water to make a slurry. Shake the bucket gently so the slurry stays loose. Insert the thermometers into the ice & water mixture so that about 2/3 of the thermometer is submerged. If it is an electronic sensor and it is waterproof, make sure it is completely submerged. Electronic devices may have a separate procedure for calibrating – check the instructions or their web site.

Let the thermometers sit for twenty minutes. Pull the thermometers out one by one and immediately note the temperature. In a mixture of water and ice, the temperature should be 32F. If not, mark a correction factor on the thermometer. If it is more than 2-3 degrees off, you probably should not use it for something as critical as frost management.

Thermometers should be placed in the low spots in the field that get the coolest. They should be away from and preferably up-wind from buildings, blacktop, or anything that might radiate heat at night. The thermometer should be about four feet off the ground and not be exposed to the sky – a simple “shelter” consisting of a board to attach the thermometer to with a cover extending 6-8 inches over the top works. Position the thermometer horizontally with the bulb is one half inch lower than the other end. Face the shelter north so that the sun never hits the thermometer. After frost season, remove them from the field and store vertically with the bulb down.

Brown Rot: A disease problem during bloom

Almond orchards are treated at least once during bloom for fungal disease brown rot. Rightfully so, as the brown rot fungi (Monilinia laxa) can kill the flower, fruiting spurs, and shoots. Flowers are susceptible from pink bud to petal fall, and are most susceptible when flowers are fully open. The fungus is able to infect all parts of the flower: anthers, pistils, petals, and stigmas (Figure 1). Upon killing the flower, it can move into and kill woodier tissues of the tree. Once in the woodier tissue, it forms a canker (Figure 2). This canker can enlarge to kill the branch and also serves as the survival structure for the fungus. These cankers are the reason why it takes many years to “clean up” an orchard after a severe brown rot infection. Brown rot can also infect the jacket of the almond and kill the young fruit as well. This is called jacket rot.

Figure 1: Blossom blight of almond caused by the brown rot fungus Monilinia laxa.
All varieties are susceptible to brown rot, but they vary in their degree of susceptibility. Butte is the most susceptible variety followed by Carmel. Ne Plus Ultra and Mission are moderately susceptible, while Nonpareil and Peerless are the least susceptible to brown rot. Brown Rot Blossom blight is usually controlled with a pink bud and full bloom treatment. Many of Brent Holtz's (UCCE San Joaquin) trials suggest that the full bloom treatment may be the most important. A third petal fall spray may be necessary in years favorable to disease (rain). If bloom is strung out and the weather is wet and rainy, no more than ten days should elapse between treatments.

A few growers have told me that they do not spray for brown rot. This may be okay if good conditions persist through bloom AND you have varieties that are not very susceptible to infection (i.e. Nonpareil). It is important to note that not spraying can lead to a serious disease epidemic: A conventional Butte orchard that missed a full bloom spray had over 50% brown rot blossom infection despite no rain!

Figure 2: Twig canker of almond caused by the brown rot fungus Monilinia laxa.
Varieties that are susceptible to Green Rot or Jacket Rot (caused by Monilina laxa, Botrytis cinerea, Sclerotinia sclerotiorum) are Butte, Ne Plus Ultra, Merced, Carmel, Price or any variety with tight clusters. Nonpareil can be affected by this disease if the right environmental conditions occur. The time of infection for Green Fruit Rot or Jacket Rot is from flower opening to petal fall.

There are several fungicides that provide excellent control of brown rot. These include the fungicide classes of Strobilurin/Boscalid (i.e. Pristine), Benzimidazole (i.e. Topsin/Benlate), dicarboximide (i.e. Rovral), anilinopyrimidine (i.e.Vanguard/Scala), and strobilurins (i.e.Abound). It is important to note that each class of fungicides should not be used in succession as rotating will help reduce the chance of resistance. An example of a two spray program could be a 5-20% bloom with Rovral+ Oil and a 100% or petal fall spray with Vanguard.

I have received a few calls about the use of Pristine for brown rot control. Pristine provides excellent brown rot control due to the fact that it contains two different fungicides, strobilurin and boscalid. These compounds were mixed to help reduce the chance of resistance. Pristine is excellent - as well as and other strobilurins (i.e. Flint/Abound/Gem) - in controlling other foliar diseases such as scab, shot-hole, anthracnose, and rust. Since strobilurins can be used to control these diseases AND the optimal timing for scab, rust, anthracnose, and shot-hole tend to be post petal fall, I would advise saving the strobilurin class (this includes Pristine) for the 2-5 week post petal fall application. Back to back applications of strobilurins should be avoided.


Please note that this is not an endorsement for any of the trade names listed, nor does the ommision of of specific trade names reflect the view of the author. Refer to your local chemical dealer or manufacturer for specific fungicide products available.

Some tasks to complete to prepare for bloom

Installing some water sensors today, I noticed that some of the Fritz were approaching pink bud. In the particular orchard, both the Fritz and Sonora look like they were ahead of the Nonpareil. Many of the Sonora's had a few blossoms either open or in popcorn. Seeing this, it is a good possibility that we may have an earlier than normal bloom - especially if the warm temperatures and sun continues. This year has provided good, early chilling in December, and enough rain to fill most soil profiles within Merced County. Both of these conditions can push bloom.

The best advice is not to jump the gun, but to keep an eye on your orchard as you work to complete complete any needed tasks.

Some tasks that should be on your list include:
1. Ensuring that frost protection strategies are in place. This includes mowing the vegetation to a few inches, starting the irrigation pumps to make sure they are working, and fixing any broken irrigation lines/emitters. Making sure that everything is working before the frost event will ensure you that it will work during the frost event (besides, who wants to troubleshoot a pump when it is 3 AM and 28 degrees outside?).
2. With rain in the forecast, it might be a good idea to make an application of a pre-emergent herbicide. This is covered in a previous article.
3. Finish shaking and flail mow any mummies on the ground to reduce Navel Orange Worm Pressure.
4. Gophers are quite active during this period and are building mounds. Traps and/or aluminum phosphide will provide control. Roger Baldwin (UCCE Specialist) suggests that combing both trapping and aluminum phosphide will increase the effectiveness of gopher control as it affects gophers that become "trap-shy." More can be found here. His article is on the third page of the newsletter.
5. Select fungicides for bloom and post-bloom sprays. Remember to rotate the fungicide's mode of action to reduce the formation of resistance. More on fungicide selection and spray timing can be found here. Click the tables to enlarge.
6. Pray for good weather during bloom!

Bloom sprays: What and when to use to provide effective disease control.

Written by Brent Holtz, UCCE San Joaquin

Almond trees are susceptible to bloom and foliar diseases when it rains at bloom, and the decision of when to spray and what fungicides to use can be quite difficult. In the San Joaquin Valley we are usually in a low precipitation region and we cannot predict when and how much it is going to rain. We often receive rain during bloom which can result in favorable conditions for several plant pathogenic fungi to cause spring time diseases of almonds. The main diseases in almonds are Brown Rot Blossom Blight, Green Fruit Rot or Jacket Rot, and Shothole. Other less familiar diseases include Scab, Rust, Leaf Blight and Anthracnose. The fungi that cause these diseases are usually always present in almond orchards, sometimes in higher or lower amounts depending on the previous year’s disease levels and current environmental conditions.

Fungicide Control Programs
Generally, a good disease control program is based upon a wise choice of fungicides and good timing and coverage. Growers should assess the diseases present in their orchards and select materials carefully. Not all fungicides are equally effective on all diseases (fig. 1). It is a good idea to use more than one kind of fungicide for a broader spectrum of activity. This will be especially important with using Strobilurin fungicides (Abound, Cabrio, Flint, Sovran, and Pristine). Pristine is a Strobilurin fungicide that is combined with Carboxyanilide to slow resistance development. Resistance to these fungicides can develop over time and repeated use, thus try to rotate the fungicides you use. Two successive applications of the same mode of action should be avoided. Information on effective fungicide rotations can be found in figure 2.

A suggested bloom spray strategy:
Usually two sprays are made for brown rot control. The first is usually done at 5-20 % bloom using a systemic fungicide. The second spray, using a different material, should be done at about 80% to full bloom or two weeks after the first spray. This is the most effective brown rot spray (fig. 3). Depending on the weather, a third spray may be necessary if rains persist and two weeks of protection have gone by. Since we cannot predict the weather at bloom time, we must at least take some

Application techniques are also important. Usually ground application is better than air; but care must be taken that both are applied correctly. In general, use properly calibrated and directed nozzles and maintain a slow speed.

Figure 1: Efficacy of fungicides for use on almond foliar and bloom diseases.


Figure 2: Suggested FRAC group rotations for the various modes of action of fungicides.

Figure 3: Treatment timings of major foliar diseases of almonds.

More information on the timing of fungicide applications for orchard tree and vine crops can be found here, at the UC IPM website.