Herbicide phytotoxicity

Possible causes

Potato crops can suffer herbicide injuries from different causes:

Seed tubers

The seed potato plants may have been accidentally contaminated during the previous year’s treatment with hormones or other herbicides (clopyralid, glyphosate, etc.) or with a sprouting inhibitor applied to the growing crops (maleic hydrazide). Another possibility is an accidental contamination of seed tubers in storage by a sprouting inhibitor (chlorpropham (CIPC)).

Soil

The soil may contain residues of the herbicide used on the previous crop, e.g. a hormone (clopyralid) overdose causing post-emergence plant deformation. This phenomenon can be observed either over the entire field or in patches (e.g. in the case of spray overlapping).

Similarly, an accumulation of these products in crop residues or in organic soil amendments (manure) may cause the same phenomenon.

Crop

• A growing crop may accidentally receive a phytotoxic product due to the application of a non-selective chemical treatment; e.g. a mistakenly used pesticide or a sprayer that has not been properly rinsed.
• The crop may receive treatment drifts from an adjacent field. In that event, the edge may be more affected than the rest of the plot (figure 1).
• A treatment overdose may induce phytotoxicity: the overdose may be due to cross-movement of spray booms or faulty treatment.

Significance

The extend of the damage is not necessarily related to the spectacular appearance of the symptoms. The consequences for the yield of herbicide phytotoxicity are better assessed by the time and duration of the symptoms rather than by their intensity.

Symptoms induced by a potato herbicide

Symptoms due to the non-selectivity of some herbicides can vary greatly and may be observed at different crop stages, usually on the foliage.

Appearance of these symptoms depends on the mode of action of the active ingredient. Several types of symptoms due to herbicides may be distinguished:

• yellow and/or white discolouration of the upper leaves; brown necroses can then form on the most affected leaves. These injuries are caused by products whose active ingredients target the photosynthesis or the production of certain pigments, e.g. metribuzine, linuron, clomazone, and aclonifen. In certain conditions, this type of herbicide may damage the crop, with symptoms often characterised by yellowing of the upper leaf tips just after emergence, followed by marking of the bottom leaves (yellow or white veins) (photos 2 and 3). The degree of susceptibility to herbicides, e.g. metribuzine, varies according to the cultivar. The application of a herbicide just prior to emergence and during cold, humid conditions, can result in plant damage.
•  foliage deformation or shrivelling occur with products whose active ingredients are designed to inhibit the enzymes responsible for the synthesis of the lipids. These herbicides usually penetrate the underground organs between germination and emergence, e.g. prosulfocarb and flufenacet (photo 4).
• mild discolouration of the terminal bunches accompanied by the haulm becoming compacted, are sometimes observed with products of the sulfonylurea family, e.g. rimsulfuron.

Symptoms induced by non-selective herbicides

The accidental application of non-selective potato herbicides may cause severe damage to potato crops:

• major leaf deformations including hooked or bent-stem symptoms: herbicides of the growth regulator family (hormones) applied to the crop itself or drifted over from an adjacent crop, can cause damage including stunted plants, deformed foliage and typical symptoms such as bent stems like a bishop’s crozier (photos 5 and 6). These symptoms may only appear in the next generation. Additionally, similar damage may appear on emergence or a few weeks later when there are herbicide residues in the soil.
• problems of emergence and stunted growth: certain non-selective herbicides mistakenly applied to the previous crop may cause failed sprouting (photo 7), sprout burns or abnormal sprout development (multitude of sprouts per eye) (photo 8) followed by poor emergence and stunted and/or abnormal plants (photo 9). The photos opposite show the damage to sprouting and on emergence of a batch of seed potatoes that received glyphosate on the previous crop.
• major leaf deformation and yellowing: non-selective herbicides usually cause leaf deformation, yellowing, discolouration or leaf necrosis and sometimes even total destruction of the plants (photos 10 to 13). Similarly, glyphosate, even at a reduced dose, applied to a seed crop (e.g. from residue in a carelessly rinsed sprayer) can cause plant deformation the following year (photo 14).
• deformed daughter tubers: certain herbicides can cause deformed and cracked tubers at harvest as well as a reduction in the number and grade of the tubers (photos 15 and 16).

Localisation of the symptoms

The symptoms of herbicide phytotoxicity appear in different ways and may point to the source of the problem:

• The edge of the field is affected or the border between healthy and affected area forms a wave. The injury is due to the off- target drift of a chemical treatment applied to the adjacent field/plot.
• The affected areas are located where the spraying started. The injury is due to overdosing in these areas.
• The affected areas are strips running the whole length of the field. The injury is due to overdosing in these areas perhaps caused by spray overlapping or the nozzle being wrongly adjusted.
• The affected areas form a triangle where the sprinkling started. The injury is due to leftovers from a previous treatment in the sprayer tank, the product in the boom gradually emptying itself. This is a case of the sprayers being rinsed carelessly or not at all (tank plus boom). The product responsible for the injury is not necessarily the product used for the last treatment applied with the sprayer. A product may stick to the sprayer wall and be released after several applications depending on the solvents in the sprayer, e.g. sulfonylureas and glyphosate are prone to this problem.
• The entire sprayed area is affected (excepting non-treated areas such as the edges of the field or corners). The injury is due to the chemical used.
• The affected areas are those covered by the sprayer the previous year or they are localised on the previous year’s crop residues. The phytotoxicity is due to herbicide persistence, which manifests itself especially in dry years (insufficient degradation of the product) and usually in light soils. No ploughing may encourage herbicide persistence.

As chemical dosages per hectare have been very significantly reduced in recent years, herbicide persistence is becoming increasingly rare.

Control

To minimise the risk of accidents, the following measures are recommended for the use of herbicides:

• check that the product is officially authorised for potatoes and apply it as recommended (e.g. according to the type of soil) and check the degree of compatibility with other pesticides;
• with a mixture of products, follow the instructions and the recommended order of introduction into the tank: WP (wettable powder); WG (water granules); SL (soluble liquid concentrate); SC (suspension concentrate); EC (emulsifiable concentrate);
• do not treat in extreme climatic conditions: high temperatures, damp or dry soil and strong winds (to avoid the risk of drift);
• avoid overdosing and spray overlapping;
• with pre-emergence herbicides, treat after the ridges are finally prepared and prior to emergence of the plants;
• do not apply metribuzine-based products on cultivars susceptible to this active ingredient;
• make sure the sprayer has been thoroughly rinsed;
• under no circumstances should a product be left in the sprayer for several days: rinse the equipment immediately after use.

Furthermore, it is highly advisable to use certified seed potatoes and, in the case of rented land, to check which pesticide treatments were applied to the previous crops.