Kokua Line June Watanabe

FL MORRIS / FMORRIS@STARBULLETIN.COM A rust disease, or Puccinia psidii, has infested the rose apple trees along the Likelike Highway. The disease was detected in the islands on an ohia plant at a Windward Oahu nursery three years ago.

Trees dying off

Forestry experts fear that a virulent fungus killing rose apple trees on Oahu may spread to ohia trees and other vulnerable native plants

Question: While driving on Likelike Highway, Kalihi-side, Kaneohe-bound, I've noticed numerous trees that appear dead or dying along the highway. Looking across the valley, it looks like some areas are nothing but dead trees! I've written to the state Department of Land and Natural Resources, but haven't received a reply.

Would you please find out?

Answer: The defoliated rose apple trees that you've seen are infested with a rust disease (Puccinia psidii), first detected in Hawaii on an ohia plant at a Windward Oahu nursery in April 2005.

The primary Hawaii host for the fungus is the rose apple tree, although an unusually high number of host trees - about 20 - have been found here, according to Rob Hauff, Forest Health Coordinator for DLNR's Division of Forestry and Wildlife.

"The disease just goes crazy on" the rose apple, causing new leaves to die and fall off, he said.

However, the concern is more over the disease's potential impact on native plant species, such as the ohia, rather than on an introduced, invasive species like the rose apple, he said.

The rose apple can be found all over the state, especially in the wetter valleys.

"It's definitely widespread," Hauff said, noting that he's seen aerial images, especially of Maui, where "the die-back is extensive."

There's really not much that can be done about the infested rose apples, he said, although he's "amazed at the resiliency of this plant." Eventually, the trees "will succumb," but they will be replaced by competing plants, Hauff said.

So far, although the disease's impact on ohia can be seen throughout the state, "it's very incidental."

Researchers are experimenting with a fungicide, but that option is meant to be used in nurseries, where conditions seem to be very conducive to the disease, Hauff said.

"If this disease were to become virulent on ohia, there really wouldn't be much of an option for treating with fungicide because the plant is so widespread and nobody is going to allow us to aerial spray with fungicide over the mountains."

The disease is believed to have been brought here in green cuttings, such as myrtle and eucalyptus, found in floral bouquets brought in from California and Florida, and indirectly from South America.

Those cuttings are known to be susceptible to the disease.

A year ago, the state passed an interim rule that prohibited the known host species from being brought in from areas known to be infested with the disease.

However that rule is expiring this month and there's no permanent rule in effect, Hauff said.

The state Department of Agriculture is preparing a permanent rule, "but that's going to take a while." During that period, the suspected green cuttings will be able to be brought in again, "much to our chagrin," Hauff said.

"Those of us who are protecting the native forests are very concerned" that another strain might come in and impact the ohia, much as the existing strain is attacking the rose apple, he said.

The main focus now is strengthening quarantine rules to make sure the disease doesn't become more virulent here, and supporting ongoing research at the University of Hawaii on controlling the disease, he said.