More answers and more questions to cotton bunchy top

22 October, 2009

A team of scientists, headed by CSIRO entomologist Lewis Wilson, are slowly unravelling the mystery behind Cotton Bunchy Top disease – a disorder reportedly costing Australian cotton growers $70 million in lost production 10 seasons ago.

Speaking on CSD’s Web on Wednesday, Dr Wilson said his team had developed a good understanding of the risk factors that affected the disease’s abundance, how the disease carried over from season to season, how it spread in-crop, and its impact on cotton yield.

The disorder sprung to prominence in the summer of 1998/99 – following a wet winter.

“Growers noticed patches of the crop where the plants were short and had short internodes, very small leaves and small bolls and these symptoms seemed to be associated with patches of aphids in the field,” Dr Wilson said.

“Initial research showed that the symptoms could be transferred from affected plants to healthy plants by grafting, strongly indicating that a disease was the cause”

“Aphids were an obvious candidate as a vector – the insect that spreads the disease – and we were able to show that cotton aphid could transfer the disease, but green peach aphid and cowpea aphid, two other species that occasionally infest cotton, couldn’t” he said.

Ten years on, the disease can still be found in low levels in fields, but has not returned to the same degree as the outbreak 1998/99 season.

Despite no widespread outbreaks of the disease since 1998/99, Dr Wilson says he and his colleagues still regularly see small areas of infected plants in commercial fields and says the risks are still there for another outbreak – if conditions were right.

“One of the risk factors would be the carry-over of the disease over the winter – and this will be strongly influenced by rainfall and the resulting availability of host plants both for cotton aphids to feed on and for the disease to reside in,” he said.

“Marshmallow is a host and we suspect other malvaceous weeds will be hosts as well. We know for sure that cotton is a host and we’re seeing a lot of cotton volunteer and ratoon plants on farms through winter and we can see Bunchy Top symptoms in those plants,” he said.

“We find aphids on those plants, so there we have a built-in on-farm disease carry-over – so control of these plants is something that growers do to reduce the risk from this disease,” he said.

The team at CSIRO have spent a lot of time investigating how cotton aphids infected with the disease spread it within a crop.

“We were interested in two things – how far the aphids spread from the original plant and as they are spreading, are they actually spreading the disease at the same rate,” he said.

“What we found was that if we infest a plant in the middle of a plot with cotton aphids carrying the disease, that plant will get the disease but it will take a fair while, up to four or five weeks, for those symptoms to show up,” he said.

“In the meantime, the aphids have spread quite a long way – so what I think is happening is that the colonising aphids are infecting the original plant, but for a while the plant hasn’t got the disease reproducing in it in sufficient amounts, so new aphids, the offspring of the colonisers, don’t pick up the disease and when they move to the next plant they’re not carrying it,” he said.

“Later you get the gradual spread of the disease, so the aphids are actually leaving the disease behind,” he said.

The CSIRO research found the yield impact of the disease depended on when, in their growth stage, plants were infected.

“We found that earlier infestations have a much bigger affect on yield which is what you’d expect because the growth of those plants is reduced during squaring and the early in the boll setting and filling period,” he said.

“For plants that were infected late – many of the bolls were already fully formed and mature and therefore not vulnerable to the disease, it is only the younger bolls that can be affected so the lower the overall effect on yield,” Dr Wilson said.

While the knowledge-base on cotton bunchy top disease has expanded considerably in the past decade, Lewis Wilson said there are still a lot of unanswered questions – not the least being identifying and classifying the virus.

This task is currently in the hands of Dr Marc Ellis from CSIRO Plant Industry in Canberra.

“We have isolated a virus from cotton bunchy top infected plants that is related to the agent for Blue Disease, which is a major problem for cotton production in parts of Africa, South America and South East Asia,” Dr Ellis said.

“Cotton Bunchy Top and Blue Disease have some similarity of symptoms as well as having similar mode of transmission,” he said.

Dr Ellis is also working closely with Dr Warwick Stiller from the CSIRO cotton breeding program at Narrabri.

“Varietal resistance to Cotton Bunchy Top is available, however, breeding efforts are being slowed by the difficulties in screening varieties for resistance, both in the field and the glasshouse,” Dr Stiller said.

“A molecular marker associated with the resistance would greatly enhance and speed up the breeding of Cotton Bunchy Top resistant varieties” he said.

Dr Lewis Wilson said molecular markers identifying the presence of the disease in plants would also be useful for understanding the disease epidemiology.

“We could then go and look at alternative host plants such as weeds and see which are the real risk factors in the overwintering and spread of the disease,” Dr Wilson said.