Managing Diplodia tip blight
Diplodia tip blight can kill the current year’s growth of trees of any age. Time fungicide applications to protect new growth from bud break through full candle elongation.
Diplodia tip blight (formerly known as Sphaeropsis) can kill the current year’s infected needles and stems of over 20 pine species, mainly two- and three-needle pine in particular, including Scots, Austrian and Eastern white pine. In some years with the right environmental conditions we have even found Diplodia on spruce, Douglas-fir and concolor fir. It can cause severe damage in nurseries, Christmas trees, ornamental plantings, landscapes, windbreaks, parks and forests, affecting both mature trees and seedlings.
The fungal pathogen Diplodia sapinea overwinters in pine shoots, bark and cones and infects growing and elongating shoots in the spring. The black, fruiting bodies are relatively large and can be seen easily with a hand lens and even the naked eye. These black dots will be on the needles, usually under the sheath, as well as on the stems and on the cones.
The last few rainy springs we have found Diplodia shoot severely infecting many Scots pine fields. During rain and windstorms, these spores will disperse over long distances and conditions for infection will remain favorable during rainy and humid days. Water stress and major wound events, e.g., hail/pruning/shearing/insects can lead to rapid disease development.
To reduce the chance of plants becoming infected, establish seedling beds away from infected pines growing in windbreaks and landscape plantings. Avoid placing container-grown seedlings beneath older pines for the purpose of hardening-off seedlings or overwintering containers. Remove cone-bearing trees near seedlings/nursery beds.
For windbreaks near seedling beds, select species other than pines. Match the tree to the site and minimize potential stress. Avoid using cone scales or pine needles for mulch. Irrigate in the morning, as seedlings will dry out faster.
If you had problems before with Diplodia, consider applying fungicides as the candles are expanding and then two more applications at 10-14 day intervals (bud break, half candle and full candle). This prevents the spores that are now being disseminated from infecting the succulent new growth. If, later in the summer, more than 10% of the trees have severe shoot blight, consider a spray program next spring.
Fungicide options include products with the active ingredient thiophanate-methyl, mancozeb or azoxystroblin. The product you choose will depend on the site of the application such as Christmas trees, nurseries or landscapes. Read and follow label directions.