what are the brown spots on my forsythia plant
Thank you for contacting the Toronto Master Gardeners regarding your Forsythia
From your photo, it is difficult to see the details of the spots and the surrounding area. Along with insect pests such as the fourlined plant bug, leaf spots can be caused by bacterial or fungal diseases. One of the first steps is to examine your entire plant for pests. Have you noticed any beetle-like bugs on your plant? If so it could be that your spots are caused by the fourlined plant bug.
The eggs of this pest hatch in late spring as the leaves emerge. The reddish orange nymphs feed on the upper side of the leaves for four weeks. Then molt into the greenish yellow 4 stripped adults which resemble beetles.When feeding both the nymphs and adults insert their needle like mouthparts into the leaves sucking out the chlorophyll. This feeding produces dark round sunken spots approximately 1/16-1/8′ wide. In most cases the four-lined plant bug feeding only affects the appearance of plants. The damage from this pest can be confused with leaf spot disease. Check plant carefully for any bugs, also make sure to check the stems for vertical slits of banana shaped eggs.If present, cut down infected branchess in the fall to remove eggs that may have been inserted into them. Next spring check your plant carefully for the nymphs. If the nymphs are present in large number a spray with insecticidal soap can be effective. Make sure to follow directions carefully. This article from the University of Minnesota gives more information on the fourlined plant bug.
Leaf spot diseases caused by various fungi sometimes attack forsythia. They are distinguished by yellow, brown or black dead blotches on the leaves. Often these spots come together to form larger patches of dead tissue. With any disease of this type, the best practice is to remove the affected leaves and destroy the leaf debris, and this includes leaves that fall at the end of the season because often spores or bacteria will overwinter in fallen leaves, or in twigs. Do not put the leaves in your compost but put them in the garbage. A fungicide available at your local garden center can be effective against this disease. Before spraying I would suggest that you take a leaf to a knowledgeable staff person at your local garden centre who would be able to suggest a treatment for your plant. Follow product directions carefully.
You might find one of our earlier posts Leaf Spot in Cherry Tree useful. It gives several site links and excerpts, which should provide you with necessary background, cultural practices and fungicide recommendations. Also, there are several key points on fungicide application.
August 26, 2022