Control of hoary cress (Lepidium draba L.) in strawberry production

The perennial weed hoary cress (Lepidium draba (L.) Desv.) is known as a problem weed in viticulture, however it also occasionally occurs in leaf crops, spring and winter cereals, where it typically establishes at the field margins. The weed has an extensive root system and can only be controlled mechanically by repeated hoeing, while in winter cereals, maize and oilseed rape it can be controlled either by synthetic auxins or inhibitors of the acetolactate synthase (ALS). In various special crops, such as strawberry (Fragaria x ananassa Duch.), these herbicides are not registered. Yet, the rhizome-deriving plants, which emerge in early spring, exert strong competitive pressure on the less competitive strawberry plants. In order to investigate the efficacy of the active ingredients registered for weed control in strawberries (status as of 2019), seeds of hoary cress introduced by compost were collected from a strawberry stand, grown into seed- and rhizome-deriving plants and then treated with the maximum registered field dose rate of the herbicides. For comparison, the herbicides were additionally applied to strawberries, field mustard (Sinapis arvensis L.), wild radish (Raphanus raphanistrum L.), shepherd's purse (Capsella bursa-pastoris (L.) Medik.) and field pennycress (Thlaspi arvense L.). In addition to the registered inhibitors of acetyl-CoA synthase (ACCase), the microtubule inhibitors propyzamide and pendimethalin, as well as napropamide-, clopyralid-, dimethenamid-, and flufenacetcontaining herbicides did not result in sufficient efficacy on hoary cress. Field application rates of herbicides containing the active ingredients isoxaben (inhibition of cellulose synthesis) and phenmedipham (photosystem II) resulted in efficacy rates of > 70%. The photosystem II inhibitors metamitron and the protoporphyrinogen oxidase (PPO) inhibitor pyraflufen achieved > 90% efficacy on seed-borne hoary cress. Similarly, high efficacy levels were found for the reference weeds. However, phytotoxic damage may occur on strawberries, therefore an application using spray guards is recommended. These results were additionally confirmed in dose-response experiments. However, all treatments of root-borne hoary cress with none of the herbicides mentioned resulted in insufficient control. Therefore, active ingredients registered in strawberries are only suitable for early control of newly introduced seed-borne hoary cress while rhizome-deriving hoary cress plants can only be effectively controlled with herbicides in the following crop.