Question: I have three Scot pines in a row that hide my back patio from the street. The past few years I’ve noticed more and more dead needles on the branches and on the ground. Some of the branches are brown all the way out to the tips. Do you know what is wrong and can I do anything do? K.V.
Answer: Your Scots (Scotch) pine tree has a disease called Diplodia shoot blight and canker (formerly named Sphaeropsis shoot blight and canker). Over the years I have seen numerous Scots and Austrian pines infected with this fungal disease.
Most people notice the disease when they see brown and dying branch tips that ooze a lot of resin. Sometimes the newest needles on infected branches will be of different lengths. If you look closely, you may see sunken or swollen, discolored areas (called cankers) on infected twigs. Heavy resin flow and absence of tunnels in the twigs help differentiate between Diplodia and insect damage.
Diplodia fungi live in and on infected shoots or branches, and pinecones. If you look closely, you can see small, black fruiting bodies and dark-colored spores, which look like pepper, on the cones. Because of the spores are on branches and cones, getting rid of infected plant parts are essential to fighting the disease.
During dry weather, cut out and destroy diseased branches by burning or burying. Prune six to eight inches below the point where dead resinous areas are on the branches. Between cuts, disinfect pruners by dipping them for at least 30 seconds in a 10-percent bleach solution or alcohol. Alternatively, you can spray pruners with a disinfectant containing at least 70-percent alcohol. Pine cones on and under infected trees should be gathered and eliminated by burning or burying.
Stressed trees are more susceptible to Diplodia. Water your trees during long periods of dry weather. Fertilize every three to five years if your pine trees are not growing at least six to twelve inches per year. Do not over-fertilize, especially with nitrogen. Finally, you might want to apply a fungicide containing thiophanate-methyl or chlorothalonil or a combination of both products at 14 day intervals between bud break and full shoot elongation to help prevent infections. Always read and follow all label directions. The fungicide you use should be labeled for use on pines or conifers.
Barb Larson is horticulture educator for Kenosha County University of Wisconsin Extension. Barb has a Master’s of Science in horticulture from the University of Wisconsin-Madison. If you have a plant or gardening question, email Larson at firstname.lastname@example.org or call her at 262-857-1942.