Spud Special: Plan ahead for blight control

The last few weeks has seen more rain making the completion of planting, for some, a challenge. Having said that, it is only seed so there is still time yet, and I certainly wouldn’t be planting it too deep, so as to encourage it to emerge as quickly as possible. Timing the application of herbicides has also been challenging, in some cases up to 50% emergence, although the addition of carfentrazone to the mix seems to stop any transient effects of residual herbicides. In most cases I have encouraged clients to go on earlier than normal with pre-emergence herbicides in the knowledge that crops are emerging extremely quickly this year, just over two weeks in some cases and the canopy will out-compete late emerging weeds.

My attention will be very quickly drawn towards blight control, as it has already been found under fleeced crops in Cheshire. The predominant blight strains continue to be 6_A1 and 13_A2. Both these strains seem to be particularly aggressive, but under differing conditions and neither necessarily need the criteria of a “Smith” period to rapidly multiply. Thus, my advice is not to be complacent when considering blight control, as these strains can infect and grow at temperatures below 10oC and this means that they are active earlier than old strains.

I recommend starting at rosette stage, usually with a protectant only, although I recommend that this should be mixed with a combination of cymoxanil and mancozeb in high risk situations. The protection of new growth during the rapid growth stage is crucial with a canopy capable of doubling in size over the course of a few days. The new, lush growth is particularly susceptible, so robust products should be used. There is good evidence to suggest that products such as Revus (mandipropamid) and Ranman Top (cyazofamid) move with the growing point to aid protection, although, in high risk situations I recommend the addition of products containing cymoxanil/mancozeb/propamocarb to provide “kickback” if required.

Blight control – planning forward

Control growth on Dumps

Late blight normally starts from dumps, ground keepers or gardens, with the first of these the most important and easiest managed source. Although tuber blight was not a major issue last year, it can be anticipated that more tubers will be dumped in 2014 as surpluses have to go somewhere. The best method for the control of dumps is to cover securely with heavy gauge black polythene and ensure no growth becomes exposed.

If you want to use chemical approaches to control, some sprout growth will be required before diquat or glyphosate can work effectively. By that time blight may have already have started on the foliage. Reglone (diquat) can now be used to treat haulm on dumps using a knapsack sprayer under an Extension of Authorisation for Minor Use (EAMU) arranged by the Potato Council and SRUC. Two applications are approved, with a maximum individual dose of 0.4 ml of product per square metre (same as 4 l/ha). Water volumes between 200 and 400 l/ha are acceptable, but the dump must be on a permeable surface. Growers who use Reglone for this purpose must have a copy of the EAMU authorisation (No. 20111882) from https://secure.pesticides.gov.uk/offlabels/

jsarup@spudagronomy.com

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