Septoria:How crop can make the Great Escape
Research into ways to use
fungicides more effectively
is making steady progress.
Andrew Blake relays two
DISEASE escape measurements could help improve septoria forecasts and allow sprays to be applied more efficiently, according to Darren Lovell of IACR Long Ashton.
Winter weather provides a good pointer to the likely risk from Septoria tritici. "But that risk will only be realised if conditions favour the disease during stem extension and flag leaf emergence," says Mr Lovell.
Recent LARS work has shown that at some sites and in certain seasons crops can escape the disease, even in apparently high risk years like 1994-95. It all depends on how rains disperses septoria spores during stem extension, he explains.
"When rainfall is limited, the upper leaves, which contribute most to yield, grow progressively further away from the infected ones at the base of the crop. This reduces the risk of disease spread.
"This phenomenon, termed disease escape, occurs in all varieties to some degree. But it is most marked in those which exhibit rapid stem extension."
Spark, rated 7 by NIAB for resistance, is tall with a moderate extension rate and often exhibits escape as does Cadenza (rated 5), another tall type which extends rapidly. Riband (3) and Consort (4) are short and extend slowly and so rarely demonstrate escape, says Mr Lovell.
Disease escape can also be achieved or enhanced by applying fungicides, he says. "Our experiments have shown that when fungicides are applied to coincide with the emergence of leaves four and five (GS32), subsequent leaves emerge during a period when the crop is protected. As a result the flag and second leaves emerge away from infective tissue."
Not surprisingly the effect is most obvious when compounds with a long lasting protective action, such as the newer triazoles and strobilurins, are used.
The current LARS target is to develop a simple means of measuring this disease escape and building it into a better forecasting system. Analysis of ADAS/CSL cereal disease survey data has revealed a clear link between November and December temperatures and septoria severity in crops later in the season. "Temperatures below freezing during this period are believed to slow reproduction and so reduce disease in early spring.
"The knock-on effect is to reduce disease pressure on the upper leaves in the same way that late sowing often does.
"Using this information we can produce a provisional risk assessment on a regional basis. But at present predictions based on this method must only be used to indicate likely disease risk.
"This seasons exceptionally mild winter means crops are at high risk and most growers are likely to have seen lots of septoria about. But current high levels of disease present no immediate threat to yield and provide little insight into final levels at the end of the season. That is where the latest work comes in."
Riband wheat at flag leaf emergence with septoria present on leaf 4. Dew or light rain would mean the flag leaf becoming infected.
• Disease escape concept.
• Stem extension rate the key.
• Rainfall still a factor.
• Simple in-field guide planned.