Nearly 2,500 miles of ocean separate these islands from our nearest mainland neighbor; this means species introductions to prehistoric Hawaii were very slow. In the six million years since these islands rose from the sea, only about 270 original plant
Nearly 2,500 miles of ocean separate these islands from our nearest mainland neighbor; this means species introductions to prehistoric Hawaii were very slow. In the six million years since these islands rose from the sea, only about 270 original plant species made it here on their own. They came as pollen, spores or seeds on the wind, as rafts or floating seeds on the ocean’s waves, or transported in the bodies, feathers or wings of birds. From these first plant colonists about 1,029 endemic flowering plants evolved in Hawaii.
As these plants evolved in an unthreatening environment, they lost the chemical or mechanical defenses their ancestors might have had. They adapted to our seasonal weather patterns and managed to live harmoniously with the insects, birds or small animals that made their way here similarly on wind, wave or wing. Then humans arrived bringing new threats to these fragile native plants. Rats and grazing animals, hungry insects and aggressive new plants came with people, endangering many native species and driving some to extinction.
Today, new pests and diseases arrive with shocking regularity. Native plants are particularly susceptible to attack by these new arrivals. In 2005, the Erythrina gall wasp landed and began destroying our native wiliwili as well as other non-native Erithrina species. Researchers finally resorted to importing a predatory wasp from Tanzania to control the pest but many trees had already perished.
Since 2010, another of our precious native trees has been under attack. Ohia trees in East Hawaii were dying rapidly from an unknown disease until researchers identified the fungus Ceratocystis fimbriata as the culprit in ohia wilt. This fungus clogs a tree’s sap wood, strangling its ability to transport water and nutrients. Infected trees often die within a month of showing noticeable symptoms. As Hawaii’s most widespread native tree, ohia is an ecologically and culturally important plant that grows from sea level to forests as high as 5,000 feet in elevation. These forests serve as critical habitats that many endangered native bird and insect species rely upon. Though saving these trees is crucial, a practical treatment method has yet to be found for forest-wide fungicide applications. It is estimated that thousands of acres of ohia forest are now infested with this disease.
Though the disease first appeared in lower Puna, it has now spread northward to Hilo and westward to the edge of Hawaii Volcanoes National Park. Residents in all areas of the island and throughout the state need to be aware of the symptoms of this destructive disease in order to monitor its spread.
The first signs of the disease are usually noticeable when the leaves in the crowns of trees turn yellow and subsequently brown within a few days or weeks. Dead leaves, on diseased trees typically remain on the branches for a while. The trunks of infected trees have olive brown to black stripes in their xylem and phloem. This is apparent if you slash the bark of the tree and is very obvious if you cut the tree and look at a cross section. Despite the “hop scotch” pattern of infection in a stand of trees, 100 percent of trees in the stand will likely succumb to the disease within three years.
When and if symptoms are seen they should be reported to the authors, J.B. Friday, Flint Hughes and Lisa Keith, listed with contact information the University of Hawaii College of Tropical Agriculture and Human Resources publication “Ceratocystis Wilt of Ohia” available at https://www2.ctahr.hawaii.edu/forestry/disease/ohia_wilt.html. There you will also find photographs of infected trees and their trunk wood showing the characteristic staining caused by the fungus.
Though it is not known how the disease spreads in ohia, in other plant hosts of similar fungi — including sweet potato, cacao, mango and eucalyptus — it is moved by insects, water, infected cuttings, pruning wounds or contaminated vehicles or tools. These transference methods may operate in ohia as well. The Ceratocystis fungus has also been found in soils under infected trees, meaning that soil may carry the disease as well. It appears that the fungus is most easily transferred in the bark or wood of infected trees. Ohia wood should not be transferred from East Hawaii to any other area on this island or on the neighbor islands. The disease can live in dead wood for over a year.
Currently, there is no effective way to protect ohia trees from infection or to cure trees that show symptoms. Minimizing the spread of Ceratocystis is the best defense at this point.
Tropical
gardening helpline
Kawika asks: I do a lot of pruning on my job and know I should clean my tools at least between plants. What is the best way to do that?
Answer: Cleaning and sharpening your tools periodically during a pruning job is always a good idea. Repeating those practices, as well as drying and oiling your tools before storage will keep them good as new for a long time.
When pruning any plants or plant parts you suspect are diseased, disinfecting your tools becomes mandatory to avoid spreading infections.
Though many tool cleaning techniques exist, a little information can help you choose your own way. If you want or need to prune between cuts, a cloth or ready-wipe containing a disinfecting product can make the task easy and is relatively effective. Dipping the tool for five minutes in a disinfectant will sterilize the tool completely.
Though products containing chlorine have been used traditionally, these products can cause pitting in your metal tools over time. Several other products will work that are less toxic to the environment and will still disinfect.
Rubbing alcohol diluted by half with water is quite effective. Vinegar full strength can also work.
Many household disinfectants are effective when used to sterilize tools. Lysol is probably the best but hand sanitizer, mouthwash containing alcohol and other cleaning products like Pine Sol are also effective.
Though these mild disinfectants do not need to be washed off between cuts, a thorough cleaning and drying before storage will keep the tools from rusting.
Tools used for cutting obviously diseased plants should be completely sterilized every cut or two. If you use a chainsaw, the blades should be brushed clean, and then sprayed with cleaning solution and run briefly to lubricate the chain. In areas where diseases such as ohia wilt exist, your shoes and clothes should be changed before moving to areas without disease. If you are driving a vehicle or farm equipment in areas where diseased plants are growing, they must also be thoroughly cleaned underneath and on the tires to ensure they do not to carry contaminated soil to healthy areas.
Thorough cleaning of all equipment involved in a job is a good habit that can greatly reduce the likelihood of disease transfer.
Email plant questions to konamg@ctahr.hawaii.edu for answers by certified master gardeners. Some questions will be chosen for inclusion in this column.
Diana Duff is a plant adviser, educator and consultant living on an organic farm in Captain Cook.
