Dothistroma Needle Blight on Pines
Lesions begin as water-soaked spots or bands, which turn tan, brown, or reddish-brown with an abrupt transition from green tissue to discolored tissue (shown at right). The red color in the needles is a result of the accumulation of dothistromin, a toxin produced by the fungus. The tips of the needles will become necrotic and die back to the lesion, while the needle bases usually stay green. One proposed resistance mechanism in pines involves the inhibition of fungal growth with sap. As such, some infected needles may produce oozing sap at the site of infection.
Once the fungus has penetrated the needle, hyphae grow into the plant cells allowing the fungus to spread. Dothistromin is dispersed into the tissue ahead of the hyphae, killing the plant cells, resulting in their collapse and the production of typical symptoms. Strong light can enhance symptoms caused by the toxin while shade can suppress them.
Within a few weeks after the lesions have formed, small black stromata will emerge from the infected needles (shown at right). These stromata contain the spores that will cause new infections, mainly by splashing rain or wind-driven rain. The spores germinate under moist conditions and although epidemics of this disease will develop quickly in areas of mild, moist climates, it is only a problem in Utah in localized areas or during wet springs.
The most immediate effect on trees infected with this disease is the reduction in growth in both height and diameter, although severe defoliation can also result in death. Trees younger than 10 years old are more susceptible. Controlling dothistroma by chemical sprays has proved feasible. In Utah, one application in May and another application in June of copper fungicides can prevent infection in both first-year and second-year needles of Austrian and ponderosa pine. However, ultimate control will be achieved through the use of resistant trees. Resistance to dothistroma varies widely within and among pines. Some seed from Austrian and ponderosa pines have been found that have a useful degree of resistance and have been used for plantings in the Great Plains.