Oracle, Arizona, Parasites and Pathogens

Pests, Pathogens and Parasites

Dendroctonus brevicomis LeConte: Western Pine Beetle usually breeds in and kills overly mature, slow-growing, decadent, or diseased trees and trees weakened by drought, lightning, fire, or mechanical injury. It is commonly found in forests with a mid-elevation level between 2,000 and 6,000 feet (600 to 1,800 m).

Adult beetles carry spores of a blue-staining fungus, Ceratocystis minor (Hedgcock) Hunt, in special pouch-like structures in their heads called mycangia. As the beetles chew their way through the bark, the spores of this wilt-causing fungus dislodge and begin to germinate.

In trees attacked in early or midsummer, it takes only a few weeks for the fungus to invade and block the conductive vessels of the inner bark and sapwood. Once the vessels are blocked, the foliage begins to fade, first to a pale green and then to yellow. Finally, perhaps after a year, the foliage may turn red brown. This fading is the first evidence of damage to the tree that is visible at a distance.

Any condition that results in excessive demand for moisture, such as tree crowding, competing vegetation, or sudden exposure to severe sunlight; or any condition that reduces the ability of the roots to supply water to the tree, such as mechanical root damage, root disease, soil compaction, or drought, can cause moisture stress and increase susceptibility to attack by the western pine beetle.

Cuscuta: Dodder is a parasitic plant that looks like yellow string entwining a green plant. Its roots infiltrate the stems of a host to derive nourishment. It will attach to almost any plant species and some vegetable crops can be devastated. Dodder may ensnare several adjoining plants and has been shown to transmit viral diseases.

Fifteen Dodder species are present in Arizona. The one described here is common to the Southeastern Arizona desert.

Its stems may be yellow, orange or white in color. It produces small white spherical flowers, appearing mainly July through September. Seeds are distributed by wind or by the manure of animals. Seeds can remain in the ground for up to five years before germinating. When it sprouts, the Dodder vine will find the stem of a nearby green plant, attach itself, and detach from its own seedling roots.

Eradication is achieved only by cutting the host (green) plant to the ground. Native plants will grow back. Dodder grows quickly and early eradication is best. Constant vigilance is required once a single infestation is found.

Phoradendron californicum: Desert Mistletoe is a perennial parasite occurring mostly on leguminous trees such as Acacia and Mesquite. Velvet Mesquite is especially susceptible. A partial parasite, Desert Mistletoe is capable of photosynthesizing, but its roots invade the bark of the host plant and take in water and some nutrients.

Desert Mistletoe has narrow scale-like leaves resembling a clump of grass that can be green, yellow-brown or brown. Its flowers are tiny and yellowish-green.

The fruit is a pinkish-orange, non-sticky berry that ripens between October and December. Birds feed on the berries, which turn sticky in their digestive tracts, and disperse the seeds to the bark of trees.

This parasite can kill its host tree over many years. One treatment is to cut off the branch of the tree 1' to 2' below the attachment point of the Mistletoe when the parasite is first noticed. However, the Mistletoe can reappear. Once the Mistletoe is as large as that shown in the photo, it may be necessary to cut the host tree to the ground. Both Acacia and Mesquite trees will re-sprout from stumps. There are no chemical treatments.