Phytotoxicities

Among the pesticides used in agriculture, many of them can be the source of phytotoxicity on vines. If more or less significant damage is sometimes associated with the use of insecticides or fungicides over-dosed or used as a mixture, it is those caused by herbicides which are the most frequent and the most damaging. Any organ of the vine can be affected, but the vast majority of symptoms are seen on the leaves.

Leaf symptoms

• Leaf growth and shape abnormalities  (Figures 1 to 4)
  • slower development of the youngest leaves which are particularly sensitive (Figures 1 and 2); in some cases their growth is completely blocked. Under these conditions, the plants eventually show a stunted appearance, especially if the phytotoxicity has occurred prematurely;
  • leaves partially or totally deformed, slightly serrated or more irregularly cut or sifted, mottled, blistered (figures 2 to 4) and more or less rolled up (glyphosate, 2,4-D);
  • a more or less significant modification of the habit of the plants, associated with a reduction, or even a total stop of the growth of the shoots.

• Leaf discoloration abnormalities (Figures 5 to 12)
  • greenish to livid tint of leaflets, yellowing, bleaching, anthocyaninization, dull and / or tan appearance of blade, etc. (glyphosate);
  • coloring anomaly with different intensity and distribution on the leaves - in spots, in more or less well delimited inter-vein areas (simazine), sometimes progressing to tissue necrosis, affecting the veins (diuron) - (figures of 5 to 7) or developing between them, generalizing at the blade, touching the leaves at the apex or lower on the shoots .

• But also leaf necrosis and dryness, as well as more or less localized and necrotic lesions

Symptoms on other organs

• The other organs of the vine are also sensitive, in particular the young berries which will sometimes be superficially corky, cracked, or even burst.
  
  

Remember that the use of a herbicide on or near a vineyard plot is not a completely trivial operation. The risks of causing phytotoxicity are never completely ruled out. Other pesticides, for example insecticides , fungicides , used alone or in mixture, substances such as growth regulators , fertilizers , can also be the source of phytotoxicity on this plant. In the same way, they can cause discolorations and leaf deformations in particular.

Two fairly classic phytotoxicities on vines

• Sulfur  : leaf scorch at temperatures above 32 ° C. On young leaves, interveinal areas turn yellow, brown and die. Superficial brown to black lesions, more or less suberized, form on the fruits. Ultimately, the berries may split when they grow larger.
• Copper  : multitudes of small brownish lesions on leaves and berries following cold, wet periods (Figure 12).

Some tips for identifying phytotoxicity

The origin of phytotoxicity is quite difficult to determine. Indeed, very often the producer refutes the possibility of having made an error or suffered an injury, at the origin of the damage. The study of the spatiotemporal distribution of the symptoms induced by this phytotoxicity makes it possible, in the majority of cases, to confirm the cause.

• Distribution in symptom space

The distribution of plants which have undergone phytotoxicity may vary depending on the composition of the phytotoxic compound, its mode of supply and its location on the plant.
- If the phytotoxic compound is applied to the foliage (foliar herbicide, insecticide or fungicide overdosed or applied under bad conditions, etc.), the distribution of symptoms and / or diseased plants may be: generalized and homogeneous; at the start of the line; on one side of the plants.
- If the compound is present in the soil in the form of residues (root herbicide, etc.), the distribution of affected plants will be: generalized and more or less homogeneous; randomly distributed over the entire plot.

• Time distribution of symptoms

The time between the intake of the product causing the phytotoxicity and the appearance of the first symptoms can be variable:
- very short (the cause and effect relationship is rapid), immediately after the application of a pesticide on or near the crop (in the form of spray);
- rather long in the case, for example, of a bad previous crop (previous annual or perennial crop, weeded by a residual herbicide or poorly leached following a dry winter; perennial crop weeded for several years (this situation leading to to an accumulation of product in the soil) or following the addition of manure made from straw from a weeded cereal crop.
Note that differences in sensitivity may exist between grape varieties. In addition, we also advise you to look at all the weeds still present in the crop or other plants cultivated nearby which may have suffered the same phytotoxicity and therefore express the same symptoms. If so, this partly confirms the hypothesis of a non-parasitic disease, and probably that of phytotoxicity.

• If you suspect phytotoxicity ask the following questions:

- Wasn't the previous crop weeded with residual herbicides?
- have herbicide treatments been carried out near your crop?
- did you rinse your treatment equipment well?
- do you maintain your spraying equipment well (cleaning, calibration, etc.)?
- did you use the right product at the right dose?
- have you not used it too frequently (cumulative effect)?
- did you follow the recommendations for use indicated on the packaging?
- did the treatment take place in particular climatic conditions (too high temperatures, various stresses)?
- did you not mix incompatible products or too many products? (some mixtures are likely to cause more or less necrotic spots on the vegetation, but also on young berries)
- Did the applications not take place in bad conditions (strong wind, temperatures too low or too high)?

When applying pesticides to vines, it sometimes happens that the pressures used to spray them are momentarily too great. Young and tender leaflets that have undergone such pressure can be found more or less sifted, or even torn.

• What to do following phytotoxicity?

While there is no quick fix in this situation, you can still take the following steps:
- clearly define the origin of phytotoxicity;
- prevent it from happening again;
- manage the plants normally and observe the evolution of symptoms which will not be fatal in all cases.

In fact, this will depend above all on the nature, the dose and the persistence of the product (s) in question, the stage of growth of the plants, of the grape variety. Some hopes are therefore permitted. No other specific measure can be recommended.