Phytophthora Root and Crown Rot of Walnut and Almond

Root and crown rot of fruit and nut trees have caused major losses within orchards around the world. First reported in California in 1912, Phytophthora, which means "Plant Destroyer" in latin, causes root and crown rot throughout the San Joaquin and Sacramento valley. The problem tends to be more severe in areas of clay to clay-loam series soils, which can be attributed to the higher water holding capacity and poor drainage of these soils in comparison to sandier soils.

The effect of Phytophthora spp. on root systems is often not noticed until the above ground parts of the tree show symptoms. Symptoms of Phytophthora damage are usually observed by poor tree vigor, loss of scaffold branches, and gumming or bleeding found around the tree trunk. Orchards affected by Phytophthora crown rot can suffer tree loss quickly, especially during the spring when winter-infected trees collapse during the first hot spell and during periods of high heat in which over-irrigation easily occurs. This is in contrast to tree loss that is attributed to root rot. In this case, by the time most growers recognize the symptoms, several trees are in a declining state due to a long period of infection (several years). Careful observance of the orchard can reveal problems and warrant remediation before the manifestation of severe orchard losses.
Figure 1: Overall view of a walnut tree with Phytophthora. Note the collapse of the tree from top to bottom. As the disease infects the roots or crown, flow of water to the highest branches is compromised, with upper branch dieback being the first above ground symptom.

In the orchard, symptoms of crown rot usually include cankers that are centered at the crown or lower part of the trunk. Removal of bark in affected areas will reveal brown or black sap-wood that may be gum soaked with stone fruit trees or slimy with pome and walnut trees. Cankers are clearly delineated, with very little dis-colorization in the wood that is found outside the necrotic areas. Removal of soil from around the trunk will reveal the canker’s infectious path, moving up the tree from the soil. In some cases, cankers may extend some distance up the trunk. Crown rot kills the tree through phloem damage which prevents the conduction of nutrients up the tree.Figure 2: Above ground symptoms of an almond tree with Phytophthora root rot. Note the pale, sparse foliage found throughout the canopy of the tree.

In the case of Phytophthora root rot, cankers found above the soil line are rare. In this case, the disease severely affects roots of all sizes, removing the trees ability to pull water and nutrients from the soil. Trees suffering from root rot will have black, mushy roots which can be observed upon excavation. Roots and soil may have a “rotten egg” smell. This smell, however, is not from the disease itself, but from the anaerobic conditions caused by excessive soil moisture. Tree death occurs when a substantial part of the root system is infected, causing tree collapse due to the inability to pull water and nutrients from the soil.Figure 3: Bark removal shows Phytophthora canker on walnut crown. A defined line marks the margin of infected and healthy tissue.

Isolations from California fruit and nut orchards have revealed over 16 different species of Phytophthora infecting and causing tree decline. The disease agent is typically introduced into orchards through plant material, infested soil, or contaminated irrigation water. Once a susceptible host becomes available and is infected, rapid population growth occurs, producing survival structures that are able to survive for several years, if not decades, within the soil. This process provides an opportunity for Phytophthora to develop resistance towards chemical control measures such as mefenoxum (Ridomil) and fumigation.Figure 4: An almond tree showing symptoms of Phytophthora crown rot. Bark removal shows the margin of the canker crossing the graft union and moving into susceptible scion. Scions of almond trees tend to be highly susceptible, with quick movement of the disease. Disease progresses much slower on the rootstock.

Field observations support the conclusion that wet and cold climates will increase Phytophthora loss in an orchard. This is due to these weather conditions being conducive to pathogen growth. Soils that are constantly wetted and kept at or above the soil saturation point have higher disease rates than trees watered slightly below saturation point. This can happen through winter rains or improper irrigation management. Periods of rainfall or water standing for over 24 hours can provide enough moisture for Phytophthora infection, regardless of the season. Cooler temperatures do not inhibit the growth of Phytophthora, and may increase pathogen reproduction thus increasing disease.

The first step in disease management is prevention. Disease spread can be reduced by cleaning equipment that may have been used in areas infested with Phytophthora. When planting a new orchard, choose locations that do not hold water. Plant the trees high on raised berms, being careful not to have the soil line above the graft union as the scions tend to be highly susceptible to Phytophthora. Use of resistant rootstocks is strongly encouraged.

Since water plays a key role in disease development, irrigation practices should be modified in areas at risk for Phytophthora to more frequent, shorter watering periods. Over-watering should be avoided, especially during periods of low water use by the tree. Also, irrigation sets should not exceed 24 hours. Furthermore, use water guards to prevent water from splashing on trees, employ practices that promote good water infiltration and penetration, and try to reduce soil compaction.

Even though proper water management is critical in disease prevention for healthy trees, it has a minimal effect on trees that are currently infected. Research has demonstrated the effectiveness of fall foliar applications of Phosphorous acid (NOT PHOSPHORIC ACID!). Apply 1-2 applications to trees that are fully leafed out on 2-4 week intervals. Soil applications and dormant sprays of phosphorous acid have provided some disease suppression, but use higher rates and are not as effective as foliar sprays. Phosphorous acid has only been demonstrated to control Phytophthora diseases; therefore, applications suggested for other purposes have not been proven by research and may be unnecessary.

Use of genetic resistant or tolerant rootstocks to help reduce the impacts of the disease has been an area of focus in many research trials. Within walnut, Paradox rootstock is significantly more resistant to P. cactorum, P. citrophthora, P. drechsleri, and P. megasperma, and is somewhat more resistant to P. citricola and P. cinnamomi than Northern California Black rootstock or English rootstock. Furthermore, newly developed Paradox genetic lines have demonstrated increased resistance to P. cinnamomi and P. citricola in comparison to the currently available Paradox rootstocks. In greenhouse studies, RX1 has exhibited resistance to both of these Phytophthora species, while VX211 has expressed resistance to P. citricola. These rootstocks, however, have not been thoroughly field tested. Within almond and peach, in comparison to Nemaguard and Lovell, Peach/Almond Hybrid rootstocks are more susceptible to Phytophthora, while Marianna 2624 is less susceptible towards the disease. Viking and Atlas have the same level of resistance as Nemaguard and Lovell.

Since not all Phytophthora species are the same, the performance of resistant rootstocks will vary by which species is present. Therefore, it is necessary to confirm which species of Phytophthora is present before orchard replanting. Contact your local farm advisor for sampling of suspected diseased trees before removal. For all tree crops, rootstocks provide a line of defense, but are only one piece of the puzzle. It is best to apply as many, if not all, orchard management practices in order to successfully manage Phytophthora.

Post Harvest Pruning of Mature Orchards

Pruning is one of the subjects that can have ten people explain it in ten different ways. This has to do a lot with the traditional thoughts on how an almond tree should look, the believed benefits of pruning, and personal experiences. It is one of the most important practices of orchard maintenance due to the expense and its effect on orchard yield and longevity. Therefore pruning should be considered more of a science than an art.

Pruning can be broken down into two phases: The tree training phase and the maintenance period phase. During the tree training phase, we work to establish and shape the permanent framework of the tree through primary and secondary scaffold selection. This occurs through the first three years of the orchard life. The maintenance pruning phase occurs from the 4th leaf on and is done to maintain the shape of the tree. This post will focus on pruning during the maintenance period of the orchard life.

In order to understand the principles of pruning, we need to understand the underlying principle of orchard management: We are farming sunlight. The more sunlight that is intercepted by tree leaves, the more production we can achieve within our orchard. Therefore, trees need to fill the available space as quickly as possible to achieve maximum yields. Along with this, we want to ensure orchard viability by having sufficient fruitwood to produce a sizable crop for the life of the orchard.

So, to do this, do we need to prune?

The short answer is "no." Studies by by farm advisors Roger Duncan (Stanislaus Co.), John Edstrom (Colusa Co.), Bill Krueger (Glenn Co.), and Mario Viveros (Emeritus, Kern Co.) have demonstrated that pruning does not increase yield over minimally, or "unpruned" trees (Table 1). As we can see, conventional, annually pruned trees did not yield more than "unpruned" trees in the varieties planted. Furthermore, "unpruned" tree productivity did not appear to decreased through the 21 years of the longest trial performed.

Table 1: The comparison of cumulative yields from four varieties of conventional, annually pruned and "unpruned" almond trees from four research trials in California. (Note: Unpruned trees referred to trees in which branches were moved out of neccessity due to other orchard practices, i.e. spraying and shaking).

One main point to keep in mind in the above studies is the expense of orchard pruning. Even though annually pruned and "unpruned" pruned trees performed relatively equally, the amount of money made per acre is higher in unpruned blocks. Crews still need to go through "unpruned" orchards to remove diseased, dead, broken, and/or low branches, but this takes significantly less time than annual tree pruning.

What are the benefits of pruning?

The benefits of conventional pruning appear to be minimal. Cutting healthy limbs out of trees to maintain "shape" usually ends up decreasing fruit wood and yields. Making large cuts also open the tree up to infection by the tree pathogens Botryosphaeria, Ceratocystis, Eutypa, and Phytophthora - especially when done near a rain event.

What is meant by "unpruned"?

Growers should limit pruning to cutting away branches that need to be removed out of necessity. These reasons include:
- Equipment access (shakers, sprayers, etc.),
- Safety for the tractor driver,
- Reducing foliar diseases including Alternaria, Hull Rot, Rust, etc.,
- Removing Dead or diseased limbs from the orchard,
- Reducing sticks at harvest.

Only by knowing your orchard and its peculiarities will you know the proper amount of material to prune. The decision to prune should not be done just because of tradition or orchard looks, but because of the benefits that it can provide to the orchard and, most importantly, your checkbook.

For more information on pruning and spacing, please check out the following research reports and presentations:
"Almond Tree Pruning by the Numbers," By Roger Duncan, UCCE Stanislaus Co.
"Integration of Tree Spacing, Pruning, and Rootstock Selection for Efficient Almond Production," Project Leader: Roger Duncan UCCE Stanislaus Co.

Almond Articles from News Media

Here are this weeks news articles related to almonds that I could find:
Western Farm Press:
Ground Water Challenges for Almonds, written by Gabriele Ludwig, Almond Board of California.

Western Fruit Grower:
Going Organic, written by David Eddy, Senior Western Editor. Contains interviews with Farm Advisors across the state.
"Leading Off: Don't Clean Up," written by David Eddy, Senior Western Editor.

California Farm Bureau "Ag Alert:"
"Trial and Error Research Benefits Organic Growers," written by Bob Johnson. Article discusses Nickel's Field Estate organic production trial. Interviews with UC Farm Advisors.

Inspecting the Orchard During the Post-Harvest Period for Damage

Shaking has almost been completed throughout many almond orchards. Through this process, a lot of stress has been placed on orchard trees. Trees may have also been damaged throughout the duration of the growing season or during the harvesting process. After harvest is completed, it is helpful to walk the orchard and observe for problematic areas.


Things to look for:
Shaker or other mechanical damage that may lead to infection by Ceratocystis,
Mite damage or webbing in trees,
Areas containing a large amount of stick-tights,
Excessive leaf defoliation due to either mite infestation or lack of water,
Gumming of the trunks or scaffolds,
Areas affected by hull-rot,
and/or areas of poor nut pick-up.

To find out how to prevent this problems, look for patterns. Does the damage appear in any specific locations, such as at the end of irrigation runs, near irrigation valves, areas of different soil textures, or near orchard borders? Were shaker damaged trees shook at the beginning or end of the day? Is only one variety affected?

Identifying these patterns will provide valuable clues about what went wrong and allow corrective practices for the coming year.

Taking a harvest sample for orchard IPM check-up

The collection and crack-out of a harvest sample is often forgotten during the bustle of the harvest. It is, however, one of the most important practices to employ as it provides a “progress report” of your season’s IPM program. A sampling of 500 nuts from orchard blocks that underwent differing pest management practices should be collected after shaking but before sweeping. These nuts can be stored in a refrigerator for 2-3 weeks until time is available to shell the kernels and inspect for insect damage. This sample will indicate what pests are in the orchard to help determine the management activities for the next year. Furthermore, it will serve as a record to compare with the huller/processor grade sheet.

Insects that cause economic damage to almond kernels include navel orange worm (NOW) (Picture 1-2), ants (Picture 3), oriental fruit moth (OFM) (Picture 4), and peach twig borer (PTB) (Picture 5), and leaffooted plant bug (Picture 6). NOW actively chews and bores through kernels, often leaving behind webbing and excessive white frass. Ants feed on the kernel skin and can hollow out kernels, leaving behind piles of “sawdust” like frass. The OFM and PTB cause similar damage on almonds, causing shallow grooves on the kernel. The only difference is that OFM leaves behind small amounts of reddish brown frass. Damage by the leafooted pant bug is noted by dark spots on the kernels.

Picture 1: Navel Orange Worm Damage to a mature almond.

Picture 2: Webbing and excessive frass left by navel orange worms.

Picture 3: "Sawdust-like" frass left by ants.

Picture 4: Damage by Oriental Fruit Moth. Note the reddish frass left behind.

Picture 5: Peach Twig Borer damage found on almond kernels. Note the lack of frass and shallow grooves.


Picture 6: Darkened spots caused by the leaf-footed plant bug.

Almond Board of California'a September edition of the Outlook E-Newsletter is available

The e-newsletter can be found here. Topics to note regarding almond production deal with reduction of Salmonella populations within the orchard, a USGS report about declining ground water and aquifers, and post harvest care of almonds for handlers.

Moderate/strong El Nino predicted for winter 09/10

According to National Oceanic and Atmospheric Administration's webpage, a moderate El nino is predicted for this coming winter. The weather pattern also has the potential to strengthen over the coming months, and is expected to last through the winter.

This is great news for many valley farmers AND dwellers as it can alleviate SOME of the current water issues. These issues include increased soil salt levels from groundwater irrigations and other farm operations, reduced dam capacity and increased environmental regulations from three drought years, and increased reliance upon groundwater which is dropping the pumping depth of many aquifers. A solution to these problems can be found in plentiful winter rains. Lets hope for adequate rains in the valley, a heavy snow pack in the mountains, and a great melt in the spring.