Tropical Gardening: Warm, wet weather ideal for disease

  • Photo courtesy of VOLTAIRE MOISE Warm, wet weather creates conditions for fungi to flourish. This colorful mushroom is helping break down wood, creating humus — but don’t eat it.

Be alert! We have fungus among us, as well as bacteria, viruses and other disease-causing organisms associated with the hot summer weather. We think of these primitive life-forms as being on the bottom of the food chain, but they really are on the top since some feed on higher life-forms at every level.

When it comes to the plants in our gardens, the best disease prevention measure you can take with ornamental plants is to start with healthy or disease-resistant plants.

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According to University of Hawaii plant pathologists, once an ornamental plant becomes badly diseased, it is difficult to nurse it back to health. For this reason, every effort should be made to prevent introduction of disease-causing organisms on plant material or in soil.

Plants with poor or weak growth usually are more susceptible to diseases. Good cultural and sanitation practices will help prevent trouble, such as removing diseased parts, including leaves or branches, as soon as they appear. Treat pruning wounds with a specially prepared material to stop entrance of wood-decaying organisms and wood-feeding insects.

For treatment of fungus diseased plants, fungicide applications should begin when disease development first appears and should continue as recommended by the manufacturer.

Other organisms that cause problems on plants, people and even buildings might not be diseases but are bothersome. Two found throughout the tropical world, including in Hawaii, are house mildew and green algae.

Mildew flourishes in damp places. Hawaii’s high humidity often creates conditions favorable to the growth of this unsightly problem.

Where mildew is already established on walls, remove as much as possible by scrubbing the discolored surfaces with a strong detergent in warm water. The remaining mold spores should then be killed with a treatment of household bleach applied at the rate of 1 pint per gallon of water. If the infested surfaces need repainting, the use of a mildew-resistant paint is recommended, or a standard paint to which a mildew retardant was added. Paints that provide a smooth surface also will discourage the lodging of mold spores.

Algae commonly grow on the surface of soil that is moist for periods of time. This is very common during the rainy season. These minute green plants often develop in such profusion that they form a rather thick, greenish to blackish mat. Growth of such magnitude in a turf planting is detrimental to the grass because algae actively compete with the grass for space and nutrients. In addition, if the algae mat dries, it forms a crust that retards or prevents the movement of water into the soil. If this occurs, the grass is subjected to a moisture stress directly because of the presence of the algae.

The same conditions that favor the growth of algae also favor the growth of fungi that bring on turf grass diseases. In fact, a close association was noted between frequent disease outbreaks and the presence of algae. Reducing the moisture level would be the ideal method; however, in many situations this is not possible and other control measures must be used. The use of fungicides that are effective against turf diseases and algae is an efficient method of control.

Bacteria, fungus and algae are not just bad guys. Limu is an algae, yogurt is the result of certain types of bacteria and those delicious mushrooms we enjoy incorporated in gourmet cooking are fungi.

However, prevention of disease organisms is vital and also includes keeping them out of Hawaii. Some folks get unhappy when they find they can’t bring certain plants or seeds into Hawaii, or if they can, they have to go through all kinds of red tape, fumigation or extended quarantines to get the plants through.

A few of these folks figure it is a bother to get permits and go through the proper procedures. They smuggle in a few plants thinking it won’t make any difference. This attitude couldn’t be further from the truth or more dangerous.

Florida has one of the best examples of what happens when folks get careless about clean plant introductions. Lethal yellowing, a disease of palms, killed palms by the tens of thousands there and is a threat to other areas such as Hawaii. The disease affects coconut palms and the Hawaiian fan palm. The disease also attacks other types of palms.

The disease, similar to a virus, is a mycoplasma that infects the plant. Taking palms from infested areas to an area free of lethal yellowing is extremely dangerous. A leafhopper is involved in spreading this palm plague. Florida lost 90 percent of its coconut palms and had to replace them with dwarf Malay coconut palms at the cost of hundreds of millions of dollars. There is no control at present except avoiding carrying this disease to unaffected areas.

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Another disease that might have been accidently introduced to Hawaii is rapid ohia death, which is thought to be caused by a Ceratocystus fungus. It is presently killing large stands of ohia in the Puna area. No cure is known, but sanitation is the key. Moving plant materials, or even wood or soil, from infected areas to healthy forests could kill most of the ohia in Hawaii.

This again shows why we should support our state and federal agricultural quarantines. Importing plants illegally could bring devastating diseases such as these to Hawaii. There are ways to bring in new plants legally. The state Department of Agriculture and USDA Plant Quarantine Office can give you the details.