Highblack dotlevels in potatoes in the
past two years should serve as a warning to growers to check this
year's field history for the disease, and consider having soil
inoculum levels tested and the use of Amistar at planting, advisers
suggest.
Levels of the fungal blemish have been higher this year, says
Paul Coleman of Greenvale
AP. Heavy rain before and after desiccation helped push
infectious spores down to the tubers, he explains. "It also meant
harvest in quite a lot of cases was delayed from three weeks to
four or five weeks after desiccation."
That extra period in the ground is critical for the development
of the disease, Jeff Peters from the
Central Science Laboratory
notes. "The amount of black dot that develops is proportionate to
the time the crop is in the ground from emergence to harvest."
Research, for example, has previously identified for Maris Piper
that the risk of black dot significantly increases after about 140
days. Results from two more varieties should be available by April,
Glyn Harper from the Potato Council's
Sutton Bridge Experimental Unit says, which should help
determine whether that effect is generic across all varieties.
But it suggests growers who know they have a high risk from
either having their soil tested or from previous field history,
should look to minimise the number of days crops, particularly
susceptible varieties, that are in the ground, Dr Peters says.
For example, for Maris Piper being grown in a high-risk soil
growers should aim for about 125 days, he advises.
But using
Amistar (azoxystrobin) as an in-furrow treatment allows growers
to adjust that time period upwards by about 10 days, he adds,
helping give extra time for bulking.
Treatment should be considered almost for any pre-pack crop for
set skin movement that will be stored, even if only for a few
weeks, that is being grown on land with potatoes in the rotation in
the past 10 years, Mr Coleman says. "Black dot seems to be getting
worse and Amistar does give a good reduction providing you do
everything else right."
That includes considering using more tolerant varieties, such as
Saxon and Vales Sovereign, burning off in good time and harvesting
promptly after three weeks, he adds.