Survey results from Papua New Guinea provide the strongest evidence so far that phytoplasmas can cause disease in bananas.
Ask a plant pathologist whether bananas get infected by phytoplasmas – a group of atypical bacteria responsible for numerous yellows-type diseases – and the answer will most likely be no. A recent paper in Australasian Plant Disease Notes does not claim to have proved that phytoplasmas cause disease in bananas (demonstrating a causal link would require an in-depth study of its own). The evidence it presents, however, should encourage plant pathologists to give phytoplasmas a second look. Richard Davis, a pathologist at the Northern Australia Quarantine Strategy and joint lead author of the paper, says he wouldn’t be surprised if more banana-associated phytoplasmas were found elsewhere in the Pacific. He believes they tend to get overlooked because once the worst-case scenarios have been eliminated, the search for an explanation is often dropped.
Davis recalls seeing unexplained wilt symptoms as part of a Papua New Guinea-Australia team of plant pathologists conducting regular surveys along the border region with the Indonesian province of Papua. “After 1999, investigating signs of wilt diseases on bananas had become a high priority because of the discovery in Papua of the blood disease bacterium and of the tropical race 4 Fusarium fungus. Every time we saw a banana plant with wilt symptoms, we would screech to a halt and split open its pseudostem, after getting permission from the farmer, of course. The internal symptoms were clearly not those of Fusarium wilt or blood disease, but we also couldn’t see any other reason to explain why those bananas were sick.”
It’s not until 2008 that a banana plant was checked for phytoplasmas using the diagnostic PCR molecular test. The sick cooking banana was growing near a stand of declining coconut trees in which a phytoplasma had just been found. When the banana sample came back positive, the search for similarly ailing banana plants was widened. Between 2009 and 2010, Davis and his colleagues surveyed six provinces of Papua New Guinea collecting samples of vascular tissues from sick and healthy banana plants.
After excluding the samples that had deteriorated too much during transport, 18 samples from 16 symptomatic plants and 2 of their suckers tested positive for phytoplasmas, whereas 12 samples from as many symptomless plants came out negative. The specific regions of phytoplasmal DNA that had been amplified were then sequenced and compared with the sequences of other phytoplasmal species. The scientists have yet to identify the majority of the phytoplasmal isolates extracted, but they think that five of them could belong to a new ‘Candidatus Phytoplasma’ species tentatively named Banana wilt associated phytoplasma (BWAP).
According to Davis, the consistent association observed between symptoms (discontinuous streaks and necrotic pockets inside the pseudostem) and phytoplasmas pretty much rules out the possibility that their presence was due to chance. “Phytoplasmas are transmitted by insects, such as leafhoppers. Given enough time, banana plants that are not regularly uprooted are likely to pick up phytoplasmas from insects feeding on them.” Davis doesn’t think that BWAP is an opportunistic plant dweller, but proving that it is a banana pathogen is complicated by the nature of phytoplasmas.
Causation is traditionally demonstrated by applying Koch’s postulates, named after the German physician who formulated them. The postulates specify that the pathogen must be isolated from the diseased organism, grown in pure culture and cause the disease when injected into a healthy organism. The problem with phytoplasmas is that nobody has yet found a way to culture them. “Phytoplasmas live exclusively in the phloem of plants or the body of their insect vectors”, explains Davis. “The upside is that they cannot be transmitted through cutting tools, because they explode when they are exposed to the outside world.” To get around the inability to culture phytoplasmas, scientists conduct controlled cage trials in which phytoplasma-free plants are exposed to insect vectors that were given infected plants to feed on.
Figuring out the banana-phytoplasma-insect pathosystem will require patience and dedication. “It cannot be done with the occasional field visit”, says Davis. “You need scientists based in the field, preferably an entomologist and a plant pathologist working together, living and breathing the disease.”
The link with coconuts is also intriguing. DNA sequence analysis shows that BWAP falls within the group of phytoplasmas implicated in major lethal diseases of coconuts elsewhere in the world. It also seems identical to the phytoplasma discovered in coconuts of Papua New Guinea, at least based on the limited sequence data publicly available. Indeed, two of the five BWAP isolates come from banana plants growing near coconut disease outbreaks, suggesting that bananas could be an alternative host to the coconut phytoplasma. The other three, however, were found near healthy coconut stands. The authors speculate that the apparent lack of transfer might be explained by as-yet-unidentified differences within the BWAP group, or by differences in the feeding behaviour of insect vectors present.
Scientists have their work cut out for them. All they have to do now is convince donors to fund them!