April 14, 2003

Drought kills lots of things, but not necessarily problem crop diseases like Fusarium crown rot and Ascochyta blight which can cause major damage in wheat and chickpeas respectively.

According to Tamworth New South Wales (NSW) Agriculture pathologist Kevin Moore, pathogens often survive longer in crop residues during drought. Weeds that harbour diseases are harder to kill and soil water and nitrogen may be out of balance.

" Often the year after drought has average to above average rainfall but, unfortunately, seasons that are good for yield are often conducive to disease," Dr Moore says.

" The challenge for advisers is to capture the benefits of a good year while minimising the risk of serious disease."

Dr Moore says most growers would be facing four paddock scenarios in the 2003 winter cropping season:
- those planted to a break crop ­ chickpea, canola or faba bean ­ in 2002;
- not planted at all in 2002 ­ " enforced long fallow",
- planted to a cereal crop in 2002 that went through to harvest, or
- planted to a cereal crop that failed in 2002.

Paddocks that had a break crop in 2002, harvested or failed, and those that had the enforced long fallow should grow wheat in 2003 with lower risk of crown rot, while paddocks that saw cereal harvested in 2002 should go to a break crop this year.

Options for paddocks that had failed cereal crops in 2002 would vary depending on seasonal conditions expected in 2003 and when the 2002 cereal crop failed.

If 2003 was conducive to crown rot ­ with dry conditions post-tillering ­ these paddocks should be under a break crop. If 2003 did not favour crown rot ­ with wet conditions post-tillering ­ growers should plant Sunco Lang or Baxter wheat varieties, or perhaps Kennedy if root lesion nematode was a major concern.

"While the incidence of crown rot is linked to the amount of infected residue, yield loss is related to moisture stress post-flowering," Dr Moore says.

"Trials at Tamworth showed that all of the stubble and fallow management practices common in central and northern NSW reduced the incidence of crown rot infected plants in the succeeding crop substantially compared to continuous zero-tillage wheat.

"The trials also support the findings of other researchers that stubble burning reduces the incidence of crown rot but does not eliminate the disease.

"They also highlight that incidence, severity and yield loss from crown rot result from a complex relationship between the pathogen and the host plant which is heavily influenced by interactions with nitrogen and soil moisture."

Dr Moore said the interactions between nitrogen and soil moisture were also complex and in some paddock situations the dry summer of 2002-2003 was likely to have resulted in high nitrogen and low moisture levels in soil, increasing the risk of severe crown rot.

Growers looking to plant chickpea should not assume that, because dry conditions over the last two seasons had not favoured Ascochyta, risk of the disease was low in 2003.

Except for Central Queensland, Ascochyta was endemic in the northern region and growers should not be complacent about it. They should remember the chickpea management plan ­ available from offices of NSW Agriculture and QDPI ­ and stick to it.