White birch trees (Betula alba) can be a beautiful addition to a landscape. However, and especially in the hot Central Valley, they will frequently suffer the symptoms of heat stress, sometimes incurring severe sunburning of the shoot tips and limbs.
Nematodes—rather than weather—are a prime contributor to the effects of heat stress in birch trees.
We’ve spent many years investigating factors associated with sunburning. In this time, we have found many issues unrelated to weather which inhibit water and mineral uptake. One of the primary culprits is the high susceptibility of these trees to plant-parasitic nematodes. Just a few examples of the species commonly extracted from the roots zone of white birch trees are (common name, species, approximate number of individuals per 250 cubic centimeter):
- Ring nematode – Criconemella xenoplax – 30
- Ring nematode – Criconemella curvata – 40
- Lesion nematode – Pratylenchus vulnus – 30
- Lesion nematode – Pratylenchus penetrans – 30
- Dagger nematode – Xiphinema americanum – 60
- Stubby root nematode – Trichodorus – 60
- Stubby root nematode – Paratrichodorus – 60
- Pin nematode – Pratylenchus – 500
All plant-parasitic species act to debilitate the root system and reduce the water and mineral absorption capacity of the tree. While infested trees may appear healthy during periods of cool temperatures, trees hosting high populations of nematodes will suffer with the onset of hot weather. With the induced water stress, tips of rapidly growing shoots exposed to sunlight will oftentimes incur mild to severe sunburning.
Nematode infestation can set off a chain reaction of infections by other insects and fungi.
Nematode damage can predispose trees to a soilborne pathogen, Verticillium wilt (Verticillium dahliae). This pathogen enters the vascular tissues of the root and moves up the tree, eventually interrupting the flow of water and nutrients. This results in wilting and death of the limb(s) fed by this vascular channel.
Once a tree is under stress, certain aromatic compounds are released from the wood which are attractive to various wood-boring insect pests. Two species commonly found in the white birch are lesser peach tree borer (Synanthedon pictipes), and carpenterworm (Prionoxystus robiniae).
When limbs are damaged, they become an easy target for opportunistic fungal pathogens, such as Botryosphaeria canker (Botryosphaeria dothidea). Although a relatively weak pathogen, once in the host tissues, the fungus becomes perennial and continues to incite disease in actively growing tissues. Another group of opportunistic pathogens which can invade the damaged tissues are wood-rotting fungi. Wood-rotting fungi can render considerable structural damages to the tree by destroying the integrity of heartwood. The result is a physically weakened tree, incapable of supporting weight or withstanding physical stress.
The primary line of defense against this chain reaction of maladies resides in preventing water stress and resultant sunburn.
This necessitates controlling the ravages of plant-parasitic nematodes. Following is a suggested list of approaches to both preventing and remediating existing maladies.
Malady: Nematode infestation and/or Verticillium Wilt
- Apply a nematicide, such as Enzone. Refer to instructions for recommended rate.
- For root nutrition, apply Synergy, 3 grams per 1000 square feet.
- Move both products into soil with irrigation water.
Malady: Wood Borers
- Apply an insecticide, such as Diazinon. Refer to instructions for recommended rate.
- Then apply a spreader, such as Break-Thru, at a rate of 2 ounces per 25 grams. Apply using a hand gun to spray open galleries to drenching.
Malady: Botryosphaeria Canker
- Prune out affected wood.
- Coat with Daconil 2787 at a rate of 3 ounces per 5 grams. Recoat prior to rains.
Malady: Wood Rot
- Coat wood with white latex paint.
- Add 4 tablespoons of Daconil 2787 per 1 gallon of paint.
- Paint wounds before wood rot enters. This includes wood borer galleries, Botryosphaeria canker sites, and sunburn wounds.
- If painting is impractical, dilute paint/Daconil mix with 2 gallons of water (i.e. 2:1 water:latex).