Poinsettia Scab Disease

Poinsettia scab is a disease caused by the fungus  Sphaceloma poinsettiae. It is a problem for greenhouse poinsettia growers in Florida and Central America and is quickly spreading to other regions. Some Euphorbia weed species are also hosts of the disease. It has been in Hawaii for many years. The disease affects both the leaves and stems; small, round lesions form on the leaves. These spots develop whitish to brown centers, have a dark red to purple rim, and often show a yellow halo. Occasionally, an infected stem will grow six inches or more above the rest of the plants. This is due to the production of growth regulating chemicals called gibberellins, produced by the fungus. The disease thrives in high humidity and wet growing conditions.  Splashing water will easily spread the spores from leaf to leaf and plant to plant. The key in controlling this disease is to stop the spread of the spores. When frequent rains occur, a cover over the plant or plants is one solution. Another approach would be to apply a protective copper fungicide that would prevent the splashed spores from starting new infections.   Infected plants will usually continue to live. However, the severity of the disease is dependent upon the amount of rainfall. Remove infected leaves, and if appropriate, prune out infected stems.  Then apply either a copper fungicide or one with the active ingredient chlorothalonil (Bravo, Daconil). As new leaves emerge, reapply the fungicide. In the future, if the disease is not severe, removing infected leaves may keep the disease under control.

Bamboo Spider Mite

The bamboo spider mite is a common pest of bamboo and will occasionally attack rice and sugar cane.  Mite damage can potentially lower the bamboo's aesthetic and economic value. The mites have a flattened body, which is straw-colored to greenish yellow with small blackish green spots.

Mite feeding will result in chlorotic (pale yellow) areas on the upper leaf surface. On the underside of the leaf, mite colonies will be found under tightly woven webbing. Generally the mites remain under the web to feed and lay eggs.

A couple of miticides, Avid and Floramite, are registered in Hawaii and are quite effective. Silicon-based surfactants such as Silwet and Silgard are recommended for use with Floramite.

 
Before buying any of the above products, however, try spraying with a soap solution or a commercial product like Safer Soap. They usually do a good job.  Make sure to spray the undersides of the leaves, good coverage is important. Come back in seven to ten days for one or more repeat applications. 

Photos: Oregon State University

Mealybugs and Aphids on the Roots

Pineapple mealybug             

 There are species of aphids and mealybugs that attack the roots and the crown line of plants. Many times when plants are pulled out of the pot, whitish fuzzy bugs in the soil or on the roots can be seen; they are the root aphids or root mealybugs.

In Hawaii, the pineapple mealybug is common. The adult females are described as plump and convex in body shape and pinkish in body color. Primarily a pest of pineapple and other bromeliads, yet these mealybugs also attack banana, celery, citrus, cotton, coffee and hibiscus. Wherever pineapple is grown, the pest is present including all of the major Hawaiian Islands. Plants rarely die from a mealybug infestation.

Control of this pest depends on controlling the ants which provide the mealybugs shelter and protection from predators and parasites. Without ants, mealybug populations are small and are slow to invade new areas because of the presence of the many natural enemies (the good guys) that are in Hawaii.

In infested areas of the garden, after the crop is removed, the soil should be turned over and all crop residue removed including grassy weeds which may harbor the mealybugs.  Host plants of this pest (listed above) should not be grown in this bed for at least a year.