March is the best time to find shamrocks for sale because of Saint Patrick’s Day. Shamrocks are members of the oxalis or wood sorrel family, which includes several hundred varieties.
Shamrocks are known for their medium sized clover-like leaves that are produced from small corms or tuberous roots. Each leaf is made of three leaflets that fold in the middle. Most members of the oxalis family fold up their leaves at night, holding them tightly to the delicate stems.
Leaves are green or purple, but flowers can be white, pink, purple, yellow or red. Some of the leaves on my shamrock have had some additional colors. The leaves first showed small yellowish areas near the edge, and then orange pustules formed on the underside of the leaves. The orange pustules are a clear description of the fungal disease called rust.
Rusts are interesting diseases because of the production of eye-catching spore bearing growths. Spore horns can have finger-like tendrils that are brightly colored and have a jelly-like texture. Galls (abnormal growths) of plant tissue are also produced two years after infection by spores.
One of the characteristics of a rust disease is that the fungus requires two different species of plants to complete its life cycle. The complete life cycle of this rust on oxalis includes corn as its alternate host. As the life cycle progresses, five different spore types develop.
Several other rust diseases are similar to oxalis rust. Cedar apple rust and Cedar-hawthorn rust also require two different species to complete the life cycle. Currently, infected red cedars and junipers have noticeable spore bearing structures. The spores will be wind blown and water splashed to apples and crabapples, where the cycle continues.
Cedar-apple rust produces brown, globular galls on the cedar tree. The galls look like dimpled golf balls ranging in size from a quarter of an inch to almost two inches in diameter. During spring rains, gelatinous bright orange spore horns are produced. The symptoms on apple leaves include small, bright orange, glistening lesions.
Chemical control of rust on oxalis is not recommended, but repotting the plant may be desirable. This disease can cause serious loss in yield and quality of sweet corn, depending on environmental conditions. Breeders are working to produce resistance in commercial sweet corn.
Control of rust in apple production includes removal of cedars within half a mile of an orchard. Fungicides can be applied periodically to help control rust if applied early for fruit and continued for leaf infection. Apple cultivars vary in susceptibility to the disease.
It may disappointing to have rust on your shamrocks, but it is interesting to watch this disease progress on other plants.
Peg Godwin is horticulture agent, Lenoir County Cooperative Extension Center. Reach her at Peg_Godwin@ncsu.edu or 252-527-2191.
