Sunday, August 12, 2012

Foliar disease can wreak havoc on pines

Dothistroma blight is a devastating foliar disease of a wide range of pine species. The causal fungus infects and kills needles. Premature defoliation caused by this fungus has resulted in complete failure of most ponderosa pine plantings in States east of the Great Plains. In the central and southern Great Plains, damages Austrian and ponderosa pines in Christmas tree, shelterbelt, and landscape plantings. In California, Oregon, and Washington, the fungus damages plantings of lodgepole and Monterey pines. Infections occur sporadically in natural stands of lodgepole and ponderosa pines in Idaho, Montana, and Washington.
The fungus has seldom been detected in young seedlings in nurseries in the United States. Yet, experience with epidemics in isolated new plantings in the central Great Plains indicates that trees infected in the nursery must have been responsible. The fungus is common on older transplants in nurseries that produce pines for landscape plantings. These nurseries are located in States.Dothistroma pini occurs in States shown below and in southwestern Alaska. Twenty pine species and hybrids are known hosts in the United States. In the Central and Eastern United States, the fungus is found most often in plantings of Austrian and ponderosa pines. These two species are highly susceptible. The fungus has not been reported in natural pine stands in the Eastern and Central United States.
Early symptoms consist of deep-green bands (below left) and yellow and tan spots on needles. The deep green color of bands does not last and cannot be detected unless observed at the onset of symptom development. Later, the spots and bands turn brown to reddish brown (below middle). The bands are brighter red and more numerous on pines in California, Oregon, Washington, and Idaho, where this disease is often referred to as the "red band" disease. Map
Figure 2Figure 3Figure 4
Early symptoms (deep green bands) on Austrian pine needles infected with Dothistroma pini. Spots and bands on Austrian bands) on Austrian pine needles infected pine needles infected with Dothistroma pini.Typical appearance of infected Austrian pine needles: needle tips brown, needle bases green.
The ends of infected needles progressively turn light green, tan, and brown, with the base of the needles remaining green (above right).Needles may develop extensive browning 2 to 3 weeks after the first appearance of symptoms. Infection is typically most severe in the lower crown (see below).
Figure 5
Dothistroma pini damage severe in lower crown.
Infected needles drop prematurely. Infected second-year needles are cast before infected current-year (first-year) needles. In some seasons, second-year needles are cast in the late fall of the year they became infected. In other seasons, loss of second-year needles is not extensive until late the following spring or early summer. Needles that become infected the year they emerge often are not shed until late summer the following year.
Successive years of severe infection result in decreased growth and, ultimately, death. The disease makes pines in landscapes unsightly and pines in Christmas tree plantings unmarketable.Dothistroma needle blight can be mistaken for brown spot disease caused by the fungus Scirrhia acicola. The symptoms on needles are similar. With both diseases, trees are affected first in the lower crown.

How to control and treat blight
Copper fungicides effectively prevent infection by D. pini. Bordeaux mixture applied twice in the growing season has provided good protection of pines in shelterbelt, Christmas tree, park, landscape, and other plantings in the Central United States. Chlorothalonil is also registered for use against D. pini. Fungicides containing copper salts of fatty and rosin acids, however, are registered for control of Dothistroma blight only in the North Central States.The first application (mid-May) protects needles from previous seasons; the second application protects current-year needles. When control is intended for plantings of Austrian or ponderosa pines, the second application can be made after considerable new growth has occurred because current-year needles of these species initially resist infection and do not become susceptible until midsummer (July).Effective control has also been obtained in plantings in the Central United States with a single application made after considerable growth has occurred (early June). There is some risk in this procedure, since infection would occur in previous years' needles before the early June application. A single application will control this disease on trees that do not have susceptible current-year needles. Many Christmas tree growers in the Central United States are effectively controlling Dothistroma with a single fungicide application.Annual spraying for control of Dothistroma blight is unnecessary in certain types of plantings. Because control of this disease can be obtained with fungicides, managers can risk not spraying in park, residential, and similar types of plantings. If infection occurs during a year in which fungicide has not been applied, fungicide can be applied the next year with confidence that good control will be obtained. If little or no infection occurs the year fungicide was not applied, spraying can be skipped for another year. On the other hand, Christmas tree growers should probably not skip spraying any years because of the high possibility of great financial loss.
Procedures for control of the disease in the Western United States will differ from those that are effective in the Central United States because of differences in the life cycle of the fungus, hosts, growth, and weather. Experience with this disease in Christmas tree plantings of shore pine in Oregon indicates that fungicide should be applied earlier than in the Central United States.The use of genetic resistance looks promising for preventing or reducing damage by this fungus. Resistant strains or clones have been identified in Austrian, ponderosa, and Monterey pines. Seed from a Yugoslavian source, which has shown high resistance, is currently used to produce Austrian pines for Great Plains plantings. Recently, several geographic sources of ponderosa pine have been identified as having high resistance. Needles of all ages are highly resistant on some trees. On other trees, current-year needles are resistant, but older needles are susceptible.

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