by Lis Friemoth
Scabs on trees, not knees
Fungal pathogen causes this problem
Almost everyone who has ornamental trees will notice spots on the leaves at some point or another during the growing season.
Each plant species has particular fungal pathogens that will cause infection, resulting in visible external symptoms. One that is particularly annoying is scab. It’s found on trees as well as gardener’s knees. And although scab can be found on many species, the pretty flowering crabapples are the trees that get a lot of homeowner notice.
Scab, it’s kind of an unusual name for a plant disease, right? Well, the interesting thing about scab diseases is that they are all characterized by, more or less, defined areas of overgrown tissue — scabs, if you will. The affected areas will change in appearance from initial infection through final loss of tissue. Early stages are usually seen as spots that become sunken as necrosis (dying of tissue); it progresses and then scabs over. After that, affected leaves will lose that dead tissue, resulting in holes.
If this fungus attacks fruit after the sunken stage, the spots will become discolored, turning from somewhat fuzzy or russetted to almost cork-like in texture.
In the case of scab, the pathogens are present in the plant naturally but re-infection occurs more quickly during the damp spring weather, and with greater severity, when infected debris is left to over-winter around the plants. The exposure time necessary for infection varies with the temperature — in general, a lower air temperature requires a longer exposure with continuous moisture and higher air temperatures.
Genera that can be plagued by some species of fungus causing scab are Beta (beet), Brassica (rutabaga), Carya (pecan, hickory), Cucurbita (squash), Cucumis (melons), Citrus, Daucus (carrot), Malus (apple, crabapple), Pastinaca (parsnip), Pisum (pea), Prunus (cherry, peach, nectarine, almond), Pyrus (pear), Raphanus (radish), Salix (willow) and Solanum (potato) — just to name a few. The severity and resultant damage varies.
The first thing to do in preventing scab, is to be sure to do a bit of homework before purchasing any new plant material. While there are no plants that are completely immune to this fungal problem, there has been a lot of research and breeding done, resulting in some excellent plant materials that show resistance to serious and severe infection while producing high-quality vegetables, fruits and nuts.
Next, ask yourself some serious questions about how much you expect out of your landscape plants. The demands of your care program will run parallel with the level of perfection demanded.
The products available differ for the homeowner and the commercial applicator. They range from preventative to restorative, and can be either synthetic chemical or organic. The key for good treatment is high-quality analysis and diagnosis, appropriate plan formulation, timely application, and good coverage. Whew! All that for a bit of scab.
— Lis Friemoth is a horticulture diagnostician. Contact her at (262) 745-2904, P.O. Box 58, Springfield, WI 53176, visit online at www.thegardenhoe.com or e-mail her at gardenhoe@tds.net. Listen to Liz from 9 a.m. to 10 a.m. every third Saturday on WISN AM1130 radio. |
