Dothistroma needle blight is caused by the fungus Mycosphaerella pini. Common hosts include Austrian, ponderosa, and mugo pines.
Symptoms: Infection usually occurs first on the lower branches of the tree and spreads upward. Diseased needles drop prematurely, the older ones first. Infected needles show yellow to reddish-brown spots or bands that extend around the needle. Needles eventually die from the area banded to the tip. Dying needle tips may be evident 2-3 weeks after bands appear. After death of diseased needle tissue, small black fruiting structures of the fungus break through the epidermis. Symptoms are similar to brown spot needle blight, but Dothistroma occurs primarily on Austrian and ponderosa pine and brown spot is more common on Scots pine.
Disease development and spread: Fungal spores are released during wet weather any time during the growing season. The fungus enters needles through stomata and survives mainly on infected needles on the tree. Needles of all ages may become infected.
- Inspect trees carefully for fruiting bodies before planting, and reject diseased trees.
- Avoid planting susceptible species next to infected trees.
- Promote good air circulation by adequate spacing and weed control.
- Improve tree vigor through cultural practices such as mulching and watering, as needed.
- Do not shear trees when foliage is wet.
- If symptoms appear, apply Bordeaux mixture or other copper- containing fungicides in mid-May and 4-6 weeks later. Fungicide sprays can be discontinued when symptoms have disappeared.--
This article originally appeared in the May 5, 1993 issue, p. 60.