Worrying levels of phoma mean vigilance is vital
By Andrew Blake
PHOMA leaf spot in oilseed rape is already at unusually high levels in susceptible hybrid Pronto at most of the five sites monitored by NIAB in the farmers weekly/DuPont Phoma Focus.
"Average incidence at the same time in previous years has been nearer 5-7%, with some sites escaping infection entirely," says NIAB pathologist Jane Thomas. "This years levels at the Kent and Notts sites are 10 times greater."
Main reason is that September conditions were favourable for spore release and infection, Dr Thomas believes. "There seems to have been a rapid appearance of spots at some sites over the past fortnight.
"To prevent the fungus reaching the stem base, growers seeing equivalent levels of disease in crops should treat as soon as possible with a first application of fungicides with eradicant activity such as Punch C (flusilazole) or Plover (difenconazole)."
Growth stages are generally from three to six leaves, but there is quite wide variation due to split germination, she notes. "Overall plant size is still small at most sites reinforcing the need to spray now, because the fungus can make rapid progress to the stem base in smaller plants."
To date spotting severity is quite low – mostly 1-5 spots a plant – and is concentrated on lower leaves. "But levels could increase quickly and spread on to the upper leaves of small plants as more spores come in. So these leaves need protection as well."
Prontos phoma resistance rating is 5. Leaf spot levels on nearby plots of the more resistant variety Escort (rated 7) are still below 10%, notes Dr Thomas. "There may be a bit more time available before this variety needs treatment."
"These are dramatic levels," says DuPonts Andy Selley. "But some plants are still only at the cotyledon stage." Unlike difenconazole, which cannot be used until the six true leaf stage, flusilazole can be used at any time, he notes. *