Investigating rates and application timing of carfentrazone-ethyl (Shark) as a dormant season herbicide on strawberry

AHDB Horticulture project SF 91a evaluated the potential for using Shark as an overall dormant season treatment to selectively clean up existing weeds in plastic mulched beds. The project investigated dormant season overall application, with Shark applied at two rates (0.33 and 0.8 L/ha) and three application timings (1 December, 28 January and 8 April) to strawberries grown on plastic covered raised beds. The December and January applications did not result in any crop injury or loss of crowns. The early April applications scorched the leaf that was present at the time of spraying but the plants re-grew strongly. There was no loss of crowns, flowers or reduction in berry numbers at the time of first pick although the overall crop canopy was slightly thinner at one site. American willowherb was controlled and dandelions present at one site were scorched and effectively controlled. The lower rate was as effective as the higher rate and no residues were found on fruit from samples taken from the first pick.

Since the completion of the trial no application for EAMU has gone forward, and no other similar herbicide has become available to control germinated weed within the crop in this way. Shark is a desiccant herbicide with good activity against mallow, American willow herb, cleavers, charlock, volunteer oil seed rape, sow-thistle, small nettle, knotgrass, pale persicaria, red shank and nightshade amongst others. Many of these species pose a significant problem within strawberry crops as they germinate over winter or in the spring just prior to or during the harvest of June bearing strawberry crops and require hand weeding. Control of these weeds would make a product such as Shark enormously useful to have available for use in the strawberry crop. It is proposed to investigate the use of Shark as a post harvest application to control weeds which have built up through harvest for example polygonums and nightshade, or as a late winter application in late March before growth starts to control overwintering weeds such as mallow.

Apart from the weed control opportunities, applying a desiccant may have some effect on fungal activity either by removing infected green tissue post harvest, by directly damaging overwintering spores and resting structures or, conversely, causing certain fungal pathogens to spore (e.g. blackspot Colletotrichum sp.) which might present an opportunity for the control of these pathogens; timing fungicides after Shark application to coincide with a stimulated spore release could improve long term control.

Reducing weeds and desiccating overwintered foliage may also reduce overwintering habitats for pests and reducing the number of flowering weeds in the early spring will also reduce pollen source for western flower thrips and avoid numbers building in the early spring prior to strawberry flowers emergence.

Option 2 – Incorporating Quickdown as an additional treatment, Quickdown is a relatively new product distributed by Certis it is used as a desiccant herbicide in potatoes and for use in hops under EAMU. It contains the active ingredient pyraflufen-ethyl (26.5g/l). Pyraflufen-ethyl is aprotoporphrinogen oxidase (PPO) inhibitor which disrupts the plants photosynthetic pathways. Consequently activity is optimised under bright and sunny conditions this may make this treatment more suitable for the post harvest timing. The product is contact acting only and controls a range of broad-leaved weeds up to 4 true leaves including groundsel, cleavers, bindweed, knotgrass and chickweed therefore increasing the spectrum of control on from just using Shark. Quickdown has no activity against grass weeds. In SCEPTRE trials this product has shown some promising results in various situations and would be very interesting to include.