Friday, December 21, 2007

Greenhouse - Scab on Poinsettia

Scab can be a disease problem on poinsettia. Fortunately it does not show up each year. The following is an article on this disease.

Scab is a type of fungal disease called a “spot anthracnose”, which means that the pathogen, Sphaceloma poinsettiae, will cause tiny round spots and will be splashed about to start new infections if leaves sit wet for long periods. The disease does not appear to be widespread this year, but it is a good idea to review the symptoms so that you can keep a watchful eye on your crop.

Although scab can cause alarm because it is unfamiliar—it is not a disease that occurs every year without fail (like Botrytis!)—it is actually very nicely controlled via fungicides and cultural adjustments. The trick is to notice that it is present in the crop, so that the diseased plants may be promptly rogued out and the neighboring plants treated for their protection.

Often it is the growing areas with the highest amounts of leaf wetness where scab disease symptoms appear. Long periods of leaf wetness are needed for infection. Adjusting cultural procedures so as to reduce the length of time the foliage sits wet is important: water early in the day, use fans strategically and heat and ventilate at sunset to avoid condensation.

Among the fungicide choices, strobilurins such as Heritage and Compass alternated with Eagle, Terraguard or Strike is a strong program that conveniently guards against powdery mildew and gives some Botrytis suppression. Mancozeb-containing materials are effective but may leave high residue, so these should be used at early stages of production.

The symptoms of scab are very striking. The spots are light-colored, small, round, and blistered out from the leaf surface; often they run down a vein. There is usually a yellow halo around each spot. The fungus also infects stems, where it causes larger raised whitish oval “scabs”. Eventually these stem lesions turn brown, after the spores have matured. The most peculiar scab symptom of all is seen on plants that have stem infections, or else extremely heavily infected leaves. The shoots that have these symptoms will hyper-elongate, so that they stretch up several inches higher than the rest of the crop. This effect is due to the plant growth hormone produced by the invading fungus!

Raised oval lesions form on stems as well.

Poinsettias with scab lesions on the stems will hyperelongate. Here the (normal height) plant at the right had only moderate leaf infection, while the one on the left had “scabs” on the stem as well. Photos: Maria Tobiasz

Article extracted from "Scab on Poinsettias Again This Season" by Margery Daughtrey, Cornell University Dept. of Plant Pathology, LI Horticultural Research & Extension Center in the October 2005 edition of Northeast Greenhouse IPM notes.

No comments: