Field mustard, commonly called charlock, is an annual spring weed from the dicotyledon class, most abundant in spring.

A distinctive feature of this species is the dense, rough hairiness of the entire plant (very similar to fodder radish).

It has a short, taproot system. The stem is 10 to 60 cm tall, branched, upright, and rough-haired. Cotyledons are heart-shaped (typical for crucifers), 4 to 10 mm long, glabrous, green, with a distinctly notched tip. Lower leaves are lyrate and dark green, while upper leaves are oblong, rough-haired, and lobed-toothed. The inflorescence is a raceme of golden-yellow flowers with characteristic spreading sepals. The fruit is a narrow, multi-seeded silique with 3–5 prominent veins and a straight, long beak, containing seeds arranged in rows. Seeds are nearly spherical, smooth, reddish-brown or dark brown, with a slight shine.

Occurrence and harmfulness

Field mustard occurs in all spring crops. It prefers heavy, humus-rich soils that are high in calcium.

Its harmfulness is related to the large number of seeds produced (several thousand) and their longevity (up to 35 years), due to the oil content. It absorbs large amounts of nutrients from the soil and serves as a host for diseases and pests of cruciferous crops.

Control

Mechanical method

The long period between planting and potato emergence is a critical time (ideal conditions for weed growth), which can be used for mechanical treatments. These are carried out using available farm machinery, e.g., ridgers, light harrows, hoes, etc. Their main goal is to create optimal conditions for rapid weed germination, followed by destruction through subsequent cultivation passes. Maximum effectiveness (over 90%) is achieved when weeds are at the seedling stage, but as they grow, effectiveness drops to about 40%. This method depends heavily on weather and soil conditions. In very wet springs, mechanical control is difficult to apply. On heavily infested fields, it may have the opposite effect, as damaged couchgrass stolons may grow and branch even more. Additionally, the mechanical system is the most costly and labor-intensive due to multiple passes. It is most commonly used in small farms and organic systems. Before potato emergence, 3 to 6 treatments should be performed at 6–8 day intervals. After emergence, the number of treatments should not exceed 2–3, and any delay increases the risk of damage, pathogen transfer via sap, and excessive soil drying (especially light soils).

Mechanical-chemical method

The most efficient and widely used weeding system. It reduces the need for mechanical cultivation, which is much less than in the purely mechanical system. After planting, usually 2–3 ridging operations are performed, followed shortly before emergence by a herbicide treatment, after which no further cultivation is needed. At this stage, identifying weed species may be difficult, so the herbicide application should be based on a treatment diary kept by every professional user of plant protection products. Based on it, one can determine which weed species may pose a threat on a given field.

Chemical method

The most effective control method is the use of herbicides, which are most effective on this species when weeds are at the seedling stage; the more vigorous the growth, the lower the effectiveness. Herbicides in potatoes are applied at two main timings: pre-emergence and post-emergence. Before using a herbicide, always read its label, which contains information on application timing and doses. Some active ingredients have medium effectiveness, and their use may result in unsatisfactory control, especially if the weed population is dense or at an advanced growth stage.

On heavily infested fields, a non-selective herbicide containing glyphosate can be applied, known as the simplified system. Its success depends on the presence of weeds in the field, as glyphosate is only absorbed by leaves and cannot be applied to potato sprouts (they would be destroyed!). Even potato sprouts just below the soil surface may be damaged (see photos A, B). It is crucial to adjust application timing according to weather conditions, mainly temperature and rainfall, which often determines effectiveness. Pre-emergence herbicides applied too late, when potato sprouts are just below the soil surface and heavy rains occur, may damage the crop. Some pre-emergence products can also be applied in early growth stages when potato plants are about 5 cm tall, e.g., prosulfocarb + metribuzin.

Active ingredients for pre-emergence control of field mustard: linuron, metobromuron, metribuzin, metribuzin + chlormazon.

Post-emergence herbicides should be applied no later than the potato canopy closure stage to ensure coverage of weeds and avoid spray retention on potato plants. Due to the pre-harvest interval, post-emergence herbicides cannot be used on early harvest crops!

Active ingredients for post-emergence control of field mustard: bentazon, metribuzin, rimsulfuron. Metribuzin applied post-emergence may cause phytotoxic symptoms on some potato varieties.

Compiled by: Dr. Eng. Janusz Urbanowicz