Crops
Modalities of manifestation
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 distribution in time (date of appearance of the first symptoms and evolution) and in space (distribution of diseased plants in the plot and evolution) of the symptoms induced by this phytotoxicity allows, in the majority of cases, to confirm the cause.
- Time distribution of symptoms
- 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);
- quite long in the case, for example, of a poor 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 straw from a weeded cereal crop or from manure made from straw of the same type.
- 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 brought to the level of the foliage (foliar herbicide, insecticide or fungicide over-dosed or applied in bad conditions ...), the distribution of 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 may be:
. generalized and more or less homogeneous;
. randomly distributed over the entire plot.
The varietal sensitivity differences may occur in the zucchini and squash. So, if you grow several varieties at the same time, you may be able to observe a sectoral distribution of diseased plants and healthy plants corresponding to different 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 partially confirms the hypothesis of a non-parasitic disease, and probably phytotoxicity if other information confirms this possibility.