white seedheads of Poa show where the groundskeeper applied preemergent
Horticulture Hotline 02/19/24
By Bill Lamson-Scribner
It is time to get your preemergent products out before your summer annual weeds start to germinate. Prevention, being pro-active, and protection is the best way to deal with summer annual weeds. Where you have used a preemergent product and where you have not, is usually obvious if your timing was good.
Indian Hawthorn, Raphs or Raphiolepis have been really suffering. Raphs have been plagued with leaf spot for years. It seems like right after the Red Tips (Photinia) were virtually wiped out by Entomosporium leaf spot, the diseased began attacking the Indian Hawthorn. Indian Hawthorn and Red Tips are in the same plant family as roses (Rosaceae).
When dealing with any plant that is very susceptible to a leaf spot disease (ligustrum a big issue now, roses, Loropetalum) I like to think of several different factors:
1. Is this plant worth keeping in the landscape or should I replace it with another plant that is not susceptible to disease.
2. Is this plant worth spraying ever 14 – 28 days to keep it in my landscape? To keep an Indian Hawthorn alive, it needs a life-support system. The plant must be sprayed at least monthly or it will get leaf spot again. You would want to start spraying anytime and right now as new leaves are forming is the best time to start spraying.
3. What cultural practices can I do to help relieve the pressure of the disease? The removal of fallen foliage (sanitation) is key when dealing with a leaf spot disease. Having a healthy soil, as determined by a soil test, is also very important because a stressed plant is more susceptible to disease. Providing the correct amount of water preferably through a drip system, so the plants are not over-watered, or drought stressed. Overhead irrigation could spread the disease. Proper fertility as determined by the soil test is also important. Use nitrogen fertilizer very sparingly in small amounts because flushes of new growth are more susceptible to leaf spot. Choose “resistant” varieties such as “Olivia”. This does not mean that this plant will never get this disease; it is just more resistant to the disease. Mulch like Cotton Burr Compost will help prevent splashes of water drops that could spread the disease. Also, this mulch will lower the watering needs of the plants. Bioscience products like KeyPlex should be considered. KeyPlex will thicken the cuticle of the leaf making more resistant to the disease as well as more drought resistant. Pruning for good air movement.
4. What control products will work best on this disease? If you decide to spray, be prepared to spray often. Honor Guard, Banner, Dithane (Mancozeb), and Kocide are good choices. These products should be used in rotation and according to label. When spraying these control products, an appropriate surfactant should be used if the label recommends one.
I am in yards all the time and leaf spot diseases are the most common disease I see in our Lowcountry landscape. I usually recommend replanting the area with plants that are not susceptible to disease; however, that can always change. I can understand spraying roses all the time with all the flowers they produce. Indian Hawthorn, one flush of flowers – ah, not so much.
Always read, understand, and follow product label. The product label is a Federal Law.
