CONSIDERATION OF GYMNOCONIA NITENS AS A POTENTIAL BIOCONTROL AGENT FOR BLACKBERRY IN HAWAII
DE GARDNER1, CS HODGES2, RC ANDERSON3 and EM KILLGORE4
1Pacific Islands Ecosystems Research Center, Biological Resources Division, US Geological Survey, University of Hawai'i, Honolulu, Hawai'i 96822, USA; 2Department of Plant Pathology, North Carolina State University, Raleigh, North Carolina 27695, USA; 3Department of Botany, University of Hawai'i, Honolulu, Hawai'i 96822, USA; 4Plant Pest Control Branch, Hawai'i State Department of Agriculture, Honolulu, Hawai'i 96814, USA
Background and objectives
Rubus argutus, also known in Hawai'i as R. penetrans or prickly Florida blackberry, was introduced from the continental USA and is the most widespread species of bramble in Hawai'i, where it invades pasture, range and native forest lands. The State of Hawai'i has declared it a noxious weed. Hawai'i has no commercial raspberry or blackberry production that would be threatened by biocontrol introductions, and all introduced species of Rubus are weedy and of no value. However, two endemic Rubus spp. occur which must be protected. The rust fungus Phragmidium violaceum previously was evaluated as a biocontrol agent for R. argutus in Hawai'i , but showed no ability to infect the target. The rust fungus Kuehneola uredinis reportedly was able to control R. fruticosus bergii in South Africa, but is already present on R. argutus in Hawai'i and is not fully effective. The endocyclic rust fungus Gymnoconia nitens causes systemic infection on R. argutus in the south-eastern USA. Primocanes infected the first growing season show no symptoms. Infection becomes apparent on floricanes emerging from root sprouts in the second season. The fungus sporulates profusely on this tissue. Infected leaves become stunted and rolled from the margins, and are shed from the plant along with small infected branches. This study was conducted to determine the susceptibility of the Hawaiian population of R. argutus to G. nitens, and to determine whether the relative lack of seasonality in Hawai'i would affect the disease cycle of the fungus. It was also necessary to determine the ability of the fungus to attack the native species, and was of interest to assess its effects on the other introduced Rubus spp., as well as on R. spectabilis from the Pacific north-west of North America, the putative ancestral relative of the native Hawaiian species.
Materials and methods
Cuttings from the Hawaiian populations of R. argutus, R. ellipticus, R. rosifolius and R. glaucus (introduced species), and of the two native species, R. hawaiensis and R. macraei, were grown in a greenhouse at North Carolina State University, and were inoculated with locally collected G. nitens. After the first growing season, 5-10 inoculated plants were (i) maintained in a cold room (4-5°C) for 3 months; (ii) maintained in a heated greenhouse throughout the winter; or (iii) maintained in an unheated shed exposed to natural winter temperatures. The following spring, all plants were allowed to resprout and resume normal growth. Upon completion by the Hawai'i Department of Agriculture of a foreign plant pathogen quarantine facility in Honolulu, tests similar to those described above were carried out in that facility. The latter work concentrated on further testing R. argutus, R. ellipticus, R. spectabilis and the two native species.
Results and conclusions
Of the cold-treated plants, R. argutus, but none of the other introduced species in Hawai'i nor R. spectabilis, was found susceptible to G. nitens. However, both of the native species showed sensitivity to inoculation with the fungus, exhibiting leaf curling and contortion, chlorotic leaf flecking and puckering, stem lesions, and stem twisting. A single plant of R. macraei developed mild sporulation. Although these were responses to intense artificial inoculation, field release of the fungus in Hawai'i was not considered advisable at present .
1. Norambuena HM, Markin GP, Gardner DE, Galdames RG, 1992. Agro Sur 20, 117-123.
2. Gardner DE, Hodges CS, Killgore E, Anderson RC, 1997. Biological Control 10, 151-158.