Forsythia and Mountain Laurel have a disease
Hi! It looks like some disease is affecting both our Forsythia and Mountain Laurel. The Forsythia has several dead branches now. It bloomed yellow a few weeks ago and seemed very healthy. It only started looking bad after the bloom ended. Could it just be from it be exposed to the cold nights in the area recently or does it have a disease? If so, how can we treat it? The other question I have is about a Mountain Laurel plant that has had an on and off leaf disease that causes darkened spots on leaves and also some areas where the leaves are not there at all. What can we put on it so that it becomes healthy again? Thanks!!
Montgomery County Maryland
Hi - There are a couple of issues that can cause dieback symptoms on forsythia.
White mold is a fungal disease caused by Sclerotinia sclerotiorum and is favored by cool, wet spring weather. Early symptoms include stem cankers followed by wilting. Splitting open the hollow stems will sometimes reveal a dark structure called a sclerotium inside (it looks like a black grain of rice). Another possibility is a canker disease, https://extension.umd.edu/hgic/topics/cankers-trees-and-shrubs. We are also investigating the possibility of some type of insect borer/beetle, although at this point we do not have conclusive data on this.
Management is the same in all cases. Prune out and remove affected stems and branches. Prune to improve air circulation within the plant canopy. Forsythia is a type of shrub that benefits from renewal pruning. After flowering is the ideal time to prune these plants. A good way to manage forsythia is to prune out one third of the shrub (the old stems) each year. Here is our page on pruning, https://extension.umd.edu/hgic/topics/pruning
Mountain laurels are very susceptible to several leaf spots diseases and are not easy to grow. https://extension.umd.edu/hgic/topics/fungal-leaf-spots-shrubs
You did not mention how old the plant is and what site conditions they are located in. This plant requires an acid, cool, moist, well drained soil in full sun to partial shade but grows best in the shade. When stressed due to environmental or cultural issues such as drought, poor soils, poor drainage, etc. they are more susceptible to problems.
Many leaf spot diseases can be tough on the plants but does not kill them. The yellow leaves will fall off. Remove any affected foliage from the ground to prevent any overwintering fungal spores from reinfecting healthy foliage. Make sure mulch is no thicker than two inches in depth and keep it away from the stems. Water deeply during dry periods if possible. No chemical controls are recommended. You can prune these shrubs (ideally after flowering) which will help to stimulate new growth and more fullness.
Hi Christa, I have some more photos of the forsythia. It looks like it has white fuzz so that suggests mold. I also see these large brown prickly looking things. I have gone into prune the bush all over but am worried it may just continue happening and kill it even more. I enclosed some more photos of what I am seeing. Should we buy an anti-mold product for it?
As for the Mountain Laurel, it is on a steep hill just beside our back patio. It has plenty of drainage. It is also near many trees as our backyard is set into a forested area, so it is in partial shade/sun depending on the time of day. It has been there for a long time and it has had this same leaf disease treated in the past by the former owners. They were in their 80s when they sold us the house, but apparently bought this plant when they were a young couple before kids so it is probably about 50 years old if I had to guess. The former owners told us that this tree had a leaf disease and they hired a plant specialist to nurse it back to health even though it was difficult because it was a plant meant to so much to them. I believe that at various points it had lost many of its leaves due to the disease. I would prefer to nurse it back to health myself rather than allowing it to kick the bucket! If you had advice besides pruning, I'd appreciate it. As is there are many branches without any leaves on it and where there are leaves many or maybe most show these spots. I worry that pruning it would prune it to death!
Hi - Thank you for the additional photos. The knobby growths on the stems (in the third picture) are galls called by a bacterial disease, Pseudomonas savastanoi. For these, our recommendation is similar as for the other symptoms you are seeing -- prune out the affected stems during dry weather and discard them. https://pnwhandbooks.org/node/2709/print
The white fuzz in your other photos looks like gray mold (Botrytis) which tends to be an issue when the weather conditions are overcast, cool, and very moist, as we have had recently. It is not typically a problem in drier weather conditions. No control (other than pruning) is recommended.
For the mountain laurel, you can treat the new foliage with a copper fungicide to manage the leaf spot issue. Fungicides are not curative and will not clear up symptoms that are already present. Treat new foliage as it emerges and every two weeks in the spring (through June). The foliage is most susceptible to leaf spots during wet, cool spring weather so that the most critical type to use a protectant fungicide.