When an individual tree turns yellow compared to all of its neighbors it often indicates there is something going wrong in the root system. The yellowing often shows up at the time the weather turns hot and the compromised roots are unable to keep up with the water needs of the tree.
There are a variety of potential causes for these symptoms and they can’t all be fully explained here but I’ll comment on some of the main things we see on farm calls.
Soil borne diseases. The two most common diseases affecting the crown and root system are Phytophthora and Armillaria. Both diseases cause similar above ground symptoms: poor terminal growth, small chlorotic leaves, premature defoliation, and decreased productivity, branch dieback and ultimately, tree death.
For Phytophthora, disease severity depends upon Phytophthora species, soil type, climatic conditions and tree age. Phytophthora affects the inner bark and cambium and typical root or crown cankers will be brownish with a fairly distinct margin as the fungus advances. Removal of the outer bark often reveals brown tissue with a water soaked zonate appearance near the margin between healthy white and infected tissues if the fungus is active. Excessive soil moisture and saturation such as occurred in the two heavy storms last winter favors infection.
Armillaria mellea or oak root fungus is identified by cutting into crown or root tissue and looking for whitish fungal plaques growing between the bark and wood. Whitish fungal strands and gumming are also commonly found in infected bark. Finding rhizomorphs, fungus signs that resemble brown to black shoestrings adhering to the outer bark of infected roots is a positive confirmation of Armillaria. They develop best in moist soil.
Armillaria often produces clusters of mushrooms around the base of infected trees following rainfall from November to February. When newly planted in an infected site it usually takes about four years for a susceptible tree to show symptoms of Armillaria. Infected trees may die suddenly when the heat of summer arrives.
Another soil borne disease that can weaken trees, crown gall, is caused by the bacterium Agrobacterium tumefaciens and is relatively easy to identify. Galls are made up of undifferentiated, disorganized tissue growths on roots and/or the tree crown. Galls most often develop on root or crown tissue underground and may not be noticed.
As galls enlarge, the center of the gall dies creating a dead wood area that can be infected by wood rotting fungi. These galls and wood rots were shown to be related to 85% of the trees lost in windstorms in surveys previously conducted in the Chico area. Trees with severe crown gall infection and girdling may be stunted and may display poor growth and yellow foliage. Before almond trees succumb to crown gall itself they often topple over from structural weakening related to the wood rots.
Rootstock compatibility. Union mild etch (UME) occurs in young orchards on Marianna 2624 plum rootstock (Prunus munsoniana x P. cerasifera) when soils are too wet during the growing season. This problem occurred on varieties in an orchard that were the least compatible with the rootstock (such as Butte or Monterey) but other varieties were affected as well.
For trees on Marianna 2624, once growth was affected by UME, leaves turned pale yellow and growth stopped. When severely affected, leaves rolled and scorched on the margins, and trees defoliated. Some trees died or remained weak enough to be removed although most affected trees recovered the following year.
Krymsk 86 is a peach-plum hybrid rootstock (Prunus persica x P. cerasifera) that can sometimes behave somewhat like its plum half. Although this rootstock appears compatible with almond, a young orchard planted on heavy clay adobe soil displayed interveinal yellowing symptomatic of manganese deficiency. Micronutrient deficiencies occur in soil related areas and are more prevalent when soils are cold or too wet thus reducing feeder root activity and nutrient uptake.
When soils are too wet during the growing season due to late rains or over irrigation, young trees on Krymsk 86 may turn yellow similar to the union mild etch (UME) problem that occurs on Marianna 2624 plum rootstock. In addition to yellowing, more severe symptoms can include rolled drooping leaves and a cessation of shoot growth.
When the problem is recognized and irrigation is optimized, some affected trees will put out a new flush of growth. Other trees will remain stalled for the current season but recover over winter and resume normal growth the following year.
Vertebrates. Pocket gophers are serious pests especially in young orchards. Root damage results in a yellow, stressed canopy, and poor tree growth. Gopher girdling on the crown mimics Phytophthora root rot, oak root fungus, union mild etch on Marianna 2624 rootstock, or the yellow tree problem on Krymsk 86 rootstock. Trees die when completely girdled.
Gophers can easily kill two to four year old trees, but I’ve seen 10 year old trees girdled and killed by gophers as well. Look for missing bark and parallel tooth marks on the wood at feeding sites about 6 inches below ground where the bark has been chewed away in a girdle about 4 to 6 inches wide.
Voles, also called meadow voles or meadow mice, may move into orchards and feed on the bark of young trees at the ground surface particularly if vegetation around tree trunks offers cover and protection. Mainly a problem on first year trees, their girdling produces symptoms similar to gopher damage. Rodents are potential pests in all orchards, but they are more likely to invade orchards that provide good cover with a cover crop or where they can migrate in from rangeland or unmanaged areas.