Pruning Wound Cankers of First Leaf Almond

Figure 1: Fungal infection of a pruning
wound that occurred during first
leaf pruning on Padre

Pruning wound cankers have been observed on all major varieties and are the most damaging to orchards. Cankers that develop on the primary (Figure 1) or secondary scaffolds (figure 2) of young trees will lead to the loss of these scaffolds and eventual tree death. Sometimes infection occurs within the wind cracks found in the base of the scaffolds. Removal of the infected scaffold may be possible, but all of the diseased wood must be removed or the infection will remain. To stave off infection, proper structuring and tying of trees to reduce wind breakage, and pruning when the weather forecast is clear, is advised. Delayed spring pruning or late fall pruning to avoid the rains may be necessary. Pruning cuts should not be cut flush with the tree, but rather be made outside of the branch collar as this increases the rate of healing. The varieties Aldridge, Carmel, and Padre appear to be more susceptible than Nonpareil or Butte. This disease has been noted to have a high occurrence around rivers, sloughs, and other riparian areas. It is important to note that once the tree is infected, there are no fungicide or nutrient sprays that can cure this disease.

Figure 2: Fungal infection of a
pruning wound made on a
secondary scaffold in Padre.

Control strategies for these three diseases have been difficult to develop due to the complexity of the fungi’s lifecycle. Initial infection is thought to occur through airborne spores that move in to an orchard from a residual population. Recent research has shown that uninfected trees can harbor populations of these fungi. It is thought that the fungus has the ability to infect and survive on dead bark, remaining dormant until conducive conditions occur for tree infection. Since these fungi are found on many species within the environment, it is difficult – if not impossible – to provide adequate control through application of fungicides. Strategies that prevent infection (i.e. proper pruning, reducing excessive vigor) should be utilized to help reduce the severity of these diseases within the orchard.

Modern farming practices may make these wounds more or less susceptible than what was previously described. Previous research has shown that pruning wounds larger than 0.5 inch diameter may be susceptible for three to five days. Larger cuts are susceptible longer. Covering wounds with paints did not appear to increase wound healing, but may prevent the wound from fungal infection. Wound susceptibility and the feasibility of protecting tree wounds and trunks with fungicides are currently being researched.

Plant New Trees High!

Written by Brent Holtz, UCCE San Joaquin County

One of the worst things that can happen to young trees from nurseries is that they are planted to deep. Sometimes  they are initially planted at the right height, but then a berm is thrown up around the trees and their crown (the graft union between the scion and rootstock) is covered with the soil from the berm. I saw many diseased trees last spring that had Phytophthora root and crown infections and every one of them had their graft union below the soil line; sometimes the union was more than six inches below. If you are planting or replanting, trees should be planted high on small mounds as shallowly as possible. Planting depth after settling should be no deeper than in the nursery, and the graft union should always be well above the soil line.

Try to follow some of these planting tips:
1. Dig a hole deep enough so the roots are spread out and not cramped,
2. Plant the trees so that the nursery soil line is above the current soil line,
3. Plant the highest root a little above the soil line and then cover it with extra dirt, and
4. When planting allow for 3-6 inches of settling in the planting hole.

I have never seen trees die from being planted too high, but I have seen many trees killed by being planted too low. This is especially true in heavy soils with a high clay content. These soils have slow water percolation, drain slowly, and remain saturated longer than well drained sandy soils. The mound around the tree trunk forces excess water to drain away from the tree, thus reducing the length of time the crown is exposed to excess surface moisture. Saturated soil conditions can occur at plating if the trees are irrigated too heavily, or when a high rainfall winter and spring occur. Some orchards survive years before a wet spring kills trees that settled too deep or had their crown covered with a berm. Plant the tree right the first time!

After making a strong case for planting the trees high, there is one exception. If you are planting on Marianna 2624 plum rootstock you should plant your trees the same depth they were planted in the nursery. With this rootstock, planting trees too high will cause them to sucker from the roots. Marianna 2624 is fairly resistant to Phytophthora and it can also tolerate excess soil moisture better than other rootstocks.

It's Been Dry - Should I Irrigate? Part 2

I have had many inquiries about applying an irrigation since we have been lacking adequate (if any) rainfall. At this time of the year, with bloom 5-6 weeks away, the answer is "yes." As bloom approaches, it is important to have adequate moisture within the soil to a depth of at least 30 inches to promote root growth during the first root flush. Research in peaches suggest that this first root flush occurs about two weeks prior to bloom.

Irrigation sets should not last longer than 24 hours and should target around 0.5 - 0.75 acre inches of applied water. After the application, the water should have time to infiltrate the soil, about 3-4 days, before the next irrigation is applied. Following these practices will help reduce the saturated soil conditions that favor soilborne diseases such as Phytophthora. After each irrigation, check the depth of the added moisture. Once moisture reaches four to five feet, application of water can be discontinued. Rainfall will only help refill the soil profile. Keep in mind that at this point, we will need more than five inches of rain to refill the moisture in most soils without supplemental irrigations.

If it rains within the next two weeks, irrigate before the rain event with another 0.5 to 0.75 inches. This will help increase the effectiveness of the rainfall. If there is no rain in the two weeks, plan for another irrigation.

This may be a good time to apply a pre-emergent herbicide. An irrigation will "water in" the herbicide, but the effect will be limited to the wetting pattern of the irrigation system.

Four weeks ago, the answer to this question was a "no." Attempting a poor excuse for why I changed my mind --- I thought we would have received some rain by now.

Its been dry, should I water?

Although we have received a bit of rain this past week, some inquiries about applying a winter irrigation have been made. There is concern that since the soil is dry from lack of rains, that the trees will be stressed, negatively affecting the fruit bud.

Briefly, an irrigation in December to alleviate water stress in dormant trees is not needed in most cases. In order for water to move into the tree's branches, it needs the "pull" created by the leaves (transpiration). Without leaves, the water may move into the roots, up a few inches into the wood through capillary action, but not into the branches and buds within the mid to upper canopy of the tree.

The only exception may be in a situation of extreme drought, in which the trees have had considerable seasonal stress and complete defoliation. This occurs rarely in California.

There are other reasons to apply a winter irrigation. These include refilling the soil profile to help with irrigations scheduling in the spring, and watering in an applied pre-emergent herbicide.

Dormant Spray Considerations for Almonds

Scab lesions on green wood of almond.

Being one of the quieter times of the year, the dormant period is a prime time to assess orchard issues and begin applying strategies to reduce various diseases and insect pests. Surveying the orchard and sampling new growth and spurs can help identify problems of scab, San Jose Scale, Navel Orange Worm, and mites. If populations are high enough, dormant strategies can be applied to reduce the damage to the upcoming season’s crop.

Scab. If a scab outbreak and the corresponding mid- to late-season defoliation were experienced this past year, a dormant/delayed dormant application of copper and oil should be considered. This spray is effective in reducing and delaying sporulation of scab twig lesions, thus reducing in-season disease incidence. It should be used in combination with properly timed bloom and petal fall sprays. Based on field observations, orchards with significant scab outbreaks in 2010 that followed the above recommendations had obvious suppression of the disease in 2011, and were able to keep the leaves on the trees until late November.

Red Halos caused by San Jose Scale
feeding/infestation in almond.

Insect Pests. Spur samples should be selected throughout the orchard to determine the presence of San Jose Scale (SJS), European Red Mite (ERM), and Brown Almond Mite (BAM). Spurs should be pulled from a main scaffold with 100 spurs samples from the orchard. If 20% of the spurs selected are infested with scale, then a one-time oil application at 6-8 gallons per acre should be applied. If over 60% of the spurs are infested, the oil application should include an insect growth regulator that targets SJS. It is important to control SJS as high populations can kill spur wood, reducing crop load. Obvious signs of scale infestation include a red halo found on green tissue.

If very little scale is found, but over 20% of the spurs contain eggs of either the ERM or BAM, an oil application at a rate of 4 gallons per acre should still be considered. This application will smother mite eggs, reducing the overwintering population.

Eggs and adults of the brown almond
mite found on an almond spur.

Another consideration to make is the inclusion of an insect growth regulator such as Dimilin for control of peach twig borer (PTB). Applied with oil during delayed dormant, this is a very effective spray. If there are no other reasons to make a dormant application, sprays targeting PTB can be made in the spring time. The application of broad spectrum insecticides (pyrethroids, organophosphates) should be avoided at this time as they target beneficials that provide control of SJS, ERM, and BAM and are toxic to bees.

Finally, growers of soft shell varieties should do a mummy count of two trees per acre. The average number of mummies found will provide an estimation of navel orange worm (NOW) damage. If there are more than two mummies found per tree, it is recommended that the trees are shaken to reduce the overwintering mummy count to two mummies per tree or less. Sanitation is also recommended on hardshells as NOW has been found to overwinter in the hulls of these varieties. 

Taking advantage of the reduced work load during the dormant season to sample the orchard may prove beneficial; if any of the above problems exist, applying the needed corrective measures will help set the stage for a successful growing year. Furthermore, it could save the cost of a trip through the orchard during the winter if pests are not present.

Almond Rootstock Trial Poster for the 2011 Almond Board of California Annual Meeting

Just in case you wanted a sneak peak at one of my poster reports for the upcoming Almond Board Meeting:
ABC2011 Rootstock Trial Poster

Presentation on Irrigation Scheduling

Although it is not the season for irrigating, I though this presentation would serve as a nice review, educational tool. The talk was developed for targeted training for the California Almond Sustainability Program.
Irrigation Course Training