When growing various crops, in addition to agrotechnical methods, which remain the main ones in weed control, it is rather difficult to do without the use of herbicides. At the same time, it is important to remember that having a high biological activity, herbicides require a highly professional approach when using them.
Valentina Demidova, Researcher of the Federal State Budgetary Scientific Institution VNIIF, Candidate of Biological Sciences;
Maria Kuznetsova, Head of the Department of Potato and Vegetable Diseases, FSBIU VNIIF, Candidate of Biological Sciences
In recent years, in various regions of Russia, there have been numerous cases when the harm from the use of herbicides due to their toxic effect on the crop protected from weeds exceeds the benefit. The aftereffect of the herbicide is the effect of the remaining residues of the agent (and its metabolites), used in previous years, on the condition of the soil, cultivated and weeds. The risk of herbicide aftereffect is mainly determined by three factors: the intensity of adsorption, degradation and migration (movement). The effect of these factors depends on the soil-climatic and agrotechnical conditions, weather, as well as the properties of the drug itself.
Especially because of this, potatoes grown in large and specialized farms suffer.
Causes of toxicosis on potatoes:
- preservation in the soil of residues of potato-toxic herbicides after their application on the crop preceding the crop rotation;
- violation of the rules for the use of "potato" herbicides (metribuzin, rimsulfuron, prosulfocarb, etc.);
- the use of sprayers with herbicide residues in tanks (subject to the sensitivity of potatoes to these substances);
- airborne droplets of herbicides during processing of adjacent fields.
Most often, the toxicosis indicated in paragraphs 2-4 is associated with non-compliance with regulations and recommendations for use.
There are not so many herbicides that are used during potato growing season. These are mainly preparations from the groups of triazines (metribuzin), aryloxyalkanecarboxylic acids (MCPA), sulfonylureas (rimsulfuron), thiocarbamates (prosulfocarb). All of these active ingredients are selective and should not cause damage to potato plants. However, under certain conditions, potatoes get stressed. This can be weather factors, the sensitivity of varieties, violation of the regulations for use, the use of low-quality products, etc.
Symptoms depend on the class of active substances. Metribuzinum causes burns and inhibition of plant growth. This is especially pronounced on sensitive varieties or when applied in dry weather with further heavy precipitation (Fig. 1).
Rimsulfuron may cause yellowing or marbling of young potato leaves. Such symptoms are very similar to the manifestation of a viral infection. For this reason, its use on seed potatoes is undesirable.
Potato plants are very sensitive to herbicides, including those used in crop rotation on other crops. The most dangerous for potatoes are the herbicides used in the previous culture from group 2 (acetolactate synthase inhibitors (ALS) and group 4 (synthetic auxins).
Group 2 herbicides include widely used sulfonylureas (metsulfuron-methyl, chlorsulfuron, triasulfuron, etc.), as well as imidazolinones (imazethapyr, imazamox, etc.). Their negative effect on potatoes is associated with a change in the synthesis of some essential amino acids. Sulfonylurea residues under certain conditions can persist in the soil and damage potatoes for a long time after application. Some imidazolinones are found in soil for several years. The manifestation of the toxic effect of this group of herbicides on potato tops is diverse and depends on the varietal characteristics of the plants. Most often, endochlorosis (or yellowing of the leaves), wrinkling (or mosaic) and, as a rule, growth retardation are observed (Fig. 2). Such symptoms are very similar to the manifestation of a viral infection. For this reason, on soils with residues of herbicides, both cleaning and approbation of fields carried out in the production of seed potatoes are difficult.
Symptoms of damage to tubers by these herbicides can be different, but longitudinal cracking of tubers (often star-shaped) and cone-shaped growths when the tubers look like popcorn are the most characteristic (Fig. 3,4).
Group 4 herbicides include derivatives of phenoxyacetic, benzoic and pyridinic acids. The least persistent (i.e. long-term) of them are phenoxyacetic acid derivatives (2,4-D). The risk of damage to potatoes by benzoic (dicamba) and picolinic (clopyralid, picloram) acid derivatives is higher. So, for the manifestation of signs of toxic damage to potatoes, the presence of clopyralid in the soil is sufficient in a dose equivalent to 0,07% of the application rate in the previous year on the previous grain crop, and at 0,7% level, a significant decrease in the yield of a sensitive crop was already noted.
Unlike sulfonylureas and imidazolinones, visible damage to potatoes by dicamba, clopyralid, and picloram appears only on the aboveground part. Plants, as a result of deformation of leaf blades, become similar to fern seedlings (Fig. 5). These herbicides affect the setting and formation of growth points (eyes) in daughter tubers. Therefore, in the year of planting potatoes in soil contaminated with residues of these herbicides, usually normal-looking tops and tubers are formed, but they have a broken program for the formation of growth points. The above symptoms appear in the next year, when such tubers are used as planting material. When carrying out phytosanitary monitoring of potato plantings, we almost every year meet plants with the above symptoms in various regions of Russia.
At the same time, it is very important to distinguish the manifestation of herbicidal toxicosis from the signs of infectious diseases. The main difference between herbicidal toxicosis and the manifestation of any disease is the massive manifestation of symptoms of damage throughout the field or locally, in areas, but not on individual plants.
When planning potato plantings, the residual aftereffect of the herbicides used in the previous season should be taken into account. The rate of destruction of the active substances of herbicides in the soil depends on many factors: the amount of the preparation used, the conditions of the season (temperature, soil moisture), type of soil, amount of microbiota, etc. Arid conditions in the year of application of sulfonylurea, imidazolinones, dicamba, picloram and clopyralid increase the risks of retaining their residues in soils and damage to potatoes in the next season.
Therefore, if there are doubts about the "cleanliness" of the field, before planting potatoes, the soil should be analyzed for herbicide residues or soil biotesting using indicator plants.
In turn, herbicides used on potatoes can also have a negative effect on subsequent crops. In this regard, we conducted a number of studies aimed at studying the toxicity level of the herbicide based on metribuzin used on potatoes on spring rape, cucumber, oats and sugar beet plants.
The phytotoxicity of metribuzin was determined in 2018 and 2019 under the conditions of a growing experiment in a greenhouse chamber under controlled hydrothermal conditions.
As test plants, we used sugar beet (v. Ramonskaya odnosemyannaya 9), oats (v. Argaman Elita), cucumber (v. Edinstvo), and spring rape (v. Ratnik). Plants were grown in beakers 80 mm in diameter with a capacity of 600 cm3, filled with samples of sod-podzolic soil taken.
To set up the experiments, soil samples were taken from the field where the herbicide was not applied (control), and from the experimental plot where potatoes were grown and the herbicide (a.v. metribuzin) was applied at a dose of 0,5 kg / ha. In both years of testing, samples were taken in the spring, in April, from a depth of the arable horizon of 0-25 cm in 10 replicates.
Growing conditions for test plants: air temperature 250C (day) and 200C (night); watering the soil up to 60% of PV.
The comparative phytotoxicity of the selected soil samples due to the aftereffect of the herbicide was evaluated 28 days after sowing the cultures according to the difference in height and weight of the test plants.
According to the results of the assessment, it was found that in the studied soil samples, inhibition of all test plants was observed in the form of a weakening stem, growth retardation compared to the control (Fig. 6-9). The height of oat plants in the control was 25-35 cm, in the studied variant 20-23 cm; sugar beets 15-20 cm (control), in the studied variant 10-13 cm; cucumber 16-22 cm (control), in the studied variant 11-14 cm; spring rape 12-14 cm (control), in the studied variant 10-12 cm.
On average, over 2 years, the decrease in the mass of test plants of cucumber to control amounted to 70,8%; sugar beets - 45,0%; oats - 44,4%; spring rape - 33,1% (Table 1).
Thus, our studies have shown that there is a negative effect of metribuzin on potatoes on various crops: cucumber, sugar beets, oats, spring rape. In order to reduce damage to various crops by herbicide residues in the soil, it is necessary to perform a whole range of techniques:
- Use minimally sufficient doses of herbicides (mixed preparations or tank mixtures containing fewer “long-lived” active substances).
- Before sowing sensitive plants, carry out deep plowing.
- Apply crop rotations that reduce the risk of crop damage by herbicides.
- Use indicator plants (seeds of crops most sensitive to this herbicide):
- for herbicides from group 2 - acetolactate synthase (ALS) inhibitors (sulfonylureas and imidazolinones) - sugar beets, rapeseed;
- for group 4 - synthetic auxins (phenoxy acid (2.4D, etc.), benzoic acid (dicamba), pyridic acid (clopyralid, picloram) - beans, flax;
- for group 5 - metribuzin - cucumber, oats, sugar beets.
5. Conduct foliar feeding of damaged plants with fertilizers based on amino acids: for example, Isabion, Aminokat, etc.