Passiflora mollissima (also known as P. tripartita) is a vine of the passion fruit family (Passifloraceae),commonly known as banana poka in Hawaii. It is one of more than 30 species of Passiflora in the Islands, all of which have been introduced. The common name refers to the yellow, elongate shape of the mature fruit, superficially somewhat resembling a banana. "Poka" is a Hawaiian word with the connotation "to climb." Banana poka is an aggressive vine that is rapidly invading mid- to upper elevation native forests. It covers smothers and breaks underlying vegetation with dense mats of stems and foliage. Resource managers of natural areas, such as those of Hawaii's national parks, consider banana poka one of the highest priority forest weed species for biocontrol. Entomologists and plant pathologists from various state and federal agencies have conducted extensive exploration in Colombia, Ecuador, and Venezuela in search of potential biocontrol agents for this vine, resulting in the importation into quarantine in Hawaii several such agents for testing. Such explorations thus far have indicated that the form of P. mollissma known in Hawaii as banana poka does not appear to exist in the wild in its putative native habitats, but rather is found only under cultivation as a food source for human consumption. This discovery has led to the confusion alluded to above as to where the Hawaiian form should be placed taxonomically.
Flower and immature fruit of banana poka (left); mature fruit (right) superficially resembling a banana.
Banana poka invading native upper elevation forests. Note the contrast in presence of banana poka between a fenced area at the edge of a forest reserve, excluding cattle, and a nonfenced area (right).
Whereas testing of potential insect agents under quarantine has yielded encouraging results, field releases have been disappointing. Hyperparasites of biocontrol insects released previously for other target lepidopteran pests presumable have made subsequent releases of lepidopteran biocontrol agents, such as those for banana poka, ineffective. In addition to insects, a University of Hawaii plant pathologist has received permission to release a powdery mildew fungus from the native South American habitats that has been shown to be specific to Passiflora. Results of these releases have not yet appeared positive.
In my own work, a forma specialis of the vascular wilt fungus Fusarium oxysporum (F. oxysporum f. sp. passiflorae) was reported in the literature causing wilt of commercial plantings of Passiflora edulis form edulis (the purple fruited form) in Australia. The yellow fruited form, P. edulis form flavicarpa was reported to show resistance to the disease. Whereas the purple-fruited form is reportedly the form of primary commercial importance in Australia, the yellow-fruited form is the only passionfruit of commercial importance as a food source in Hawaii (although the purple-fruited form is occasionally grown by individual homeowners). It was of interest to test the fungus against banana poka to ascertain any biocontrol potential of this pathogen (Gardner, 1989).
Cultures (left), and macro- and microconidia of Fusarium oxysporum f. sp. passiflorae from Australia.
Seedlings (left), and older plants (right) of banana poka root-dip inoculated with Fusarium oxysporum f. sp. passiflorae, demonstrating the pathogenicity of this fungus to banana poka. Seedlings on the left side of the flat in the left photo were inoculated with the fungus, compared with control seedlings, root-dipped in water, on the right side. Likewise, the dead plants in the right photo were inocuated with the fungus, whereas the healthy plants were dipped in water. External growth of the fungus on the lower stem of the dead plants (right) can be seen (arrow).
In addition to the root-dip inoculation method, plants were grown in soil to which F. oxysporum f. sp. passiflorae had been added. In quarantine, the plants in the flat in the foreground (right) were grown in infested soil, compared with control plants in noninfested soil in the back. Upon issuance of permission to conduct greenshouse testing, larger plants on the left (right photo) were grown in infested soil, compared with those grown in noninfested soil at right.
In addition to banana poka, other weedy species of Passiflora in Hawaii were inoculated with F. oxysporum f. sp. passiflorae. Some were found susceptible, whereas were resistant. Among the susceptible species was P. ligularis.
The seedlings of Passiflora ligularis in the right pot, left photo, were root-dip inoculated with F. oxysporum f. sp. passiflorae, compared with controls in the left pot. In the right photo, a larger inoculated plant has died of the wilt disease. The fungus is growing externally on the surface of the stem (arrow).