A great frustration for agronomists and growers alike are the crop disorders which we’re powerless to control.

Verticillium Wilt is one such problem.

Crop Consultant Australia’s survey data from the 2017-18 revealed a quarter of agronomists felt Verticillium Wilt has resulted in $50 per hectare reduction in profit through either increased cost of production or reduced yield.

NSW DPI disease surveys have shown this disease incidence increased rapidly through the mid 1980’s but then decreased and plateaued until about 2010 – an effect of more resistance varieties, particularly Sicala V2.

Incidence has increased again from 2010 and while this incidence (the number of fields where the disease is present), is not as high as Black Root Rot and Fusarium in these surveys, the level of crop loss in the areas where is does occur is arguably greater that either of these diseases.

Unfortunately, the disease seems well adapted to high yielding cotton production systems.

• High nitrogen input exacerbates the disease under all irrigation systems. It’s a hard call for anyone to significantly reduce nitrogen inputs and risk yield loss from deficiency.
• Low-deficit irrigation practices, used to drive high yields exacerbates the disease.
• The overhead irrigation systems that many growers have installed in recent years for improved water use efficiency also seem to increase vulnerability. The more surface-dominant crop root systems place the plant is the zone where there is highest pathogen levels. Even with very conservative cotton rotations, such as Cotton to Wheat, several pivots throughout NSW have needed to be pulled out of cotton entirely due to the impact of the disease.
• Back-to-back cotton increases the pathogen levels in the soil. In years of good allocation, this creates a big problem for ‘land-poor’ farms.

Historically, plant breeding has formed the basis of integrated disease management strategies in cotton.  Bacterial Blight was a major problem to the industry in its infancy but dwindled to insignificance in the late 1980’s- early 1990’s with the introduction of resistant varieties.

Fusarium Wilt shot to prominence in the mid-1990’s and rendered many fields, particularly on the Darling Downs, unsuitable for cotton. Higher F-rank varieties now mean this disease is manageable.

For breeders, Verticillium wilt is proving a bigger challenge.

CSIRO Cotton Breeding Lead, Dr Warwick Stiller says by world standards our current varieties have high levels of Verticillium Wilt resistance but unfortunately, that doesn’t help those growers that are significantly impacted by the disease.

Dr Stiller contends the reasons Verticillium Wilt has proved to more difficult foe than Fusarium Wilt are because it is far more environmentally influenced than Fusarium, unlike Fusarium there are different isolates and VCGs of Verticillium that have a different host plant response and field sites for screening this disease are generally not as uniform or reliable.

“Importantly also, there are no commercial varieties around the world that have significantly better resistance than the current Australian varieties.

“All of these things go together to make breeding for resistance very challenging.

“We are working on developing germplasm that has improved resistance, but unfortunately there won’t be a breeding solution for those bad fields in the near future.

Dr Stiller expects molecular techniques will assist in the future, however, these techniques can’t improve resistance by themselves.

“These techniques only track what we have. Until we develop germplasm with increased resistance, molecular techniques have no value”, he said.

Frustrating again, varietal resistance to Verticillium wilt is temperature sensitive – so varieties with even a high V-rank will succumb to disease when average temperatures drop to 20-22 degrees Celsius or below.

What this means for those in the field is that you could have done everything right – planted a high V-rank variety, avoided excessive N levels, used a good rotation avoided over-watering – and one cool period in December to February could see significant areas defoliate or die.

Clearly this will be another problem we need to research our way out of but in the meantime, for those in the field, any weather forecast for a cold change in December – February will send an extra shiver up their spine.