Hawaii Forestry Extension

As we come to the end of , we should remember that plant pathogens are also invasive species. The myrtle rust fungus psidii (formerly called Puccina psidii) was discovered in Hawai‘i in 2005 and was probably accidentally brought in on imported plants or cut foliage. The fungus quickly spread around all the islands. The most common host in Hawai‘i is rose apple ( , photo 2), and the disease has killed rose apple trees around the state. But the pathogen also attacks many other species in the myrtle family (Myrtaceae), including Java plum (Syzygium cumini, photo 2), paperbark (Melaleuca quinquenervia, photo 3), jaboticaba (Plinia cauliflora, photo 4), and most importantly, ‘ōhi‘a (Metrosideros polymorpha, photo 5). Outbreaks of the rust in wet ‘ōhi‘a forests on O‘ahu and Moloka‘i killed tens of thousands of trees a decade ago but seem to have subsided. Rapid ‘Ōhi‘a Death is caused by two different vascular wilt fungi (Ceratocystis lukuohia and C. huliohia) that attach the tree’s sapwood, whereas the rust is a disease of the leaves. Austropuccinia rust does not seem to attack strawberry guava (Psidium cattleianum); the biocontrol that was introduced to reduce the spread of strawberry guava is a gall-forming insect (Tectococcus ovatus), not the rust fungus. Since spores of rust fungi blow in the wind, there is little that can be done to stop the spread of a rust disease once it is established. The best practice would be consistent and thorough agricultural inspection procedures to keep diseased plants out.

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Invasive species by definition cause harm to our economy, our environment, our health, or our way of life. While most invasive plants cause harm to our economy (think agricultural weeds) or our environment (think Miconia in the forest understory), albizia trees can actually kill you. These giant trees (now called falcalta, formerly Falcataria moluccana, Paraserianthes falcataria, and Albizia falcataria before that) can fall over in storms, crushing houses and cars and downing power lines. They also change the forest ecosystem to allow other invasive plants in. While our native ‘ōhi‘a does well in nutrient-poor young lava soils, albizia fixes nitrogen and enriches the soil, allowing invasive plants such as strawberry guava (Psidium cattleianum) to thrive. Here again is a way that invasive species interact, and the combined effects are worse than the sum of the individual effects.

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May is Hawai‘i Invasive Species Awareness Month but also Wildfire Community Preparedness Month. Some invasive plants, especially grasses, create the fuel that feeds our all to frequent wildfires. Fountain grass ( ), shown here, is one of the worst, but buffel grass (Cenchrus ciliaris), molasses grass (Melinis minutiflora), Kikuyu grass (Cenchrus clandestinus), and Guinea grass (Megathyrsus maximus) also add to the wildfire problem. Except for fountain grass, these grasses are all valuable forage species and have a place in our agro-ecosystems, but abandoned pastures and unmanaged lands are soon taken over by flammable grasses. Since these grasses are adapted to fires, they regenerate rapidly after each fire, while trees and shrubs disappear.
Follow for tips on how to protect your home and land against wildfires.

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Missing the old Moreton Bay Fig (banyan) that used to grace the waterfront in Kailua-Kona. The tree was cut down last year after a large branch broke off and crashed into the road and the tree was deemed a hazard. Yes, I know that that species can spread and is invading our native forests, especially at Kalōpā in Hāmākua where it is parasitizing some gigantic, old ‘ōhi‘a trees, but that tree was magnificent and gave some much needed shade. I wonder why they left the stump.

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One invasive species can multiply the effects of another. Feral pigs dig up plants and knock over tree ferns, but they also spread the fruit of strawberry guava ( ). Without any local pests or diseases, strawberry guava can create impenetrable thickets, and it is now the most common tree in Hawai‘i. A recent study found that 2/3 of the seedings or saplings in our forests are non-native plants like strawberry guava, even though about 60% of our forests are mainly native trees. In time, without intensive management, the native trees in the overstory of these badly invaded forests will die off and the forest will revert to being dominated by non-native species.

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When was the last time you saw a rose apple in flower in Hawaii? The tree ( ) was badly hit when the Austropuccinia rust disease reached Hawai’i in 2005. Most rose apple trees were badly infected (see second photo) and lost most or all their leaves. Many died altogether. The pathogen, called psidii (formerly Puccina psidii) infects many plants in the myrtle family, including ‘ōhi‘a (Metrosideros polymorpha). Plant diseases are also invasive species and can have profound effects on both agriculture and native ecosystems.

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April is . What places in Hawai‘i do you know that are named for native plants? Ilima Lane in Hilo is named for the ‘ilima plant ( fallax), a low sprawling shrub in the Hibiscus family (Malvaceae) with bright yellow flowers. The plant is common along coastlines across Hawai‘i and other Pacific islands. ‘Ilima is the flower of the island of O‘ahu and the flowers can be strung into a golden, rope-like lei consisting of hundreds of individual flowers. If you are looking for the native ‘ilima, look closely because there are at least six other naturalized species of Sida in Hawai‘i. Photos from Puʻuhonua o Hōnaunau National Historical Park, Kona, Hawaii island, Hawaii.

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April is . What places in Hawai‘i do you know that are named for native plants? Manono Street in Hilo is named for manono, which is a general Hawaiian name for trees and shrubs in the genus , formerly called Hedyotis. Manono are common in wet forests across Hawai‘i and are easily recognized by their leaf display, clusters of four-part flowers, and fruit. The genus is indigenous to Hawai‘i and also native to other Pacific islands. There is also a Manono street in Kailua, O‘ahu.Photo 1 is from Koke‘e on Kaua‘i, photo 2 is from Hāmākua.

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