Crops
Phytotoxicities on melon
* Remember that the use of a herbicide on or near a crop is never a completely trivial operation. The risks of causing phytotoxicity are never completely ruled out. Inhibitors of cell division, synthesis of fatty acids and amino acids as well as phytohormones are the most involved in leaf and root malformations in melons.
Other pesticides, for example insecticides and fungicides used alone or in mixture, substances such as fertilizers , can also be the cause of phytotoxicity on melon.
Leaves
- Leaf deformations
- slow development of the youngest leaves, which can go as far as a total stop of growth; plants eventually show a stunted appearance, especially if phytotoxicity has occurred prematurely;
- partially or totally deformed sheets (figures 1 and 2), slightly serrated or more irregularly cut (figures 3 and 4), marbled, blistered and more or less rolled up;
- the more or less marked distortion and / or winding of the leaves, which can be shorter, threadlike (figure 5) and have strongly thickened tissues;
- the corkscrew, crumpled appearance (figure 6) of all the leaves ...
- Leaf discolorations
- yellowing in spots, in more or less well defined areas, sometimes evolving towards tissue necrosis;
- yellowing of the veins and contiguous tissues (figure 7);
- yellowing of young leaves at the apex (figure 8);
- diffuse yellowing of the leaf blade between the veins (figure 9);
- yellowing of the periphery of the limbus (figure 10);
- diffuse yellowing of the entire blade;
- more or less homogeneous inter-vein yellowing of the limbus (figure 11), sometimes progressing to tissue bleaching;
- bleaching of the leaf blade ...
Other color anomalies may appear: greenish to livid tint of the leaves, dull and / or tan appearance of the leaf blade (figure 12)
- Leaf lesions and spots
- Lesions, more or less extensive chlorotic spots, quickly becoming necrotic ...
It should be noted that in certain circumstances, pesticides or fertilizers added voluntarily or inadvertently on the leaves can be responsible for local burns taking the appearance of more or less extensive spots (figures 12 to 15). Such damage can be observed in the presence of herbicide spray carried on the plants by the wind; following treatment with an anti-powdery mildew fungicide, or with a mixture of pesticides (fungicide, insecticide, etc.) and a foliar fertilizer; when adding fertilizer on the fly in the crop ...
Often, only leaves that are well exposed (for example near openings in cultivation under cover) or located on one side of the plants are affected. On these, the symptoms are particularly marked in places where there may have been a greater retention of the suspension of the product (s).
- Leaf wilting and drying out (figure 16)
Twigs, crown, roots
We have sometimes observed in many melons grown in the field swelling of their necks caused by the inappropriate use of a herbicide belonging to the chemical family of toluidines (pendimethalin) (figure 17). This same symptom is also manifested under the same conditions on other crops such as Tomato. On Melon, this takes very spectacular proportions because this species is very sensitive to herbicides. In this case, pendimethalin inhibits the development of lateral roots; the swelling is due to the numerous root forms which have stopped in their development.
Fruits
The rather fragile cuticle of the young fruits, slightly attacked by the sprayed pesticide (s), is covered with a multitude of initially oily lesions and more or less in relief (figures 18 and 19). These are often localized on the most exposed side of the fruit. When spraying is heavy, there may be an accumulation of the suspension under the fruits in contact with the plastic mulch. It is then at this level that we can see symptoms. Subsequently, these alterations heal and more or less locally give the cuticle a corky appearance (figure 20). The altered and corky tissues have lost their elasticity and more or less significant bursts occur at their level when the fruits grow bigger.
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
- 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 melon. Thus, 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.
Ask yourself the right questions!
To confirm phytotoxicity, we suggest you ask yourself at least the following questions:
- Was 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 take good care of your spraying equipment (cleaning, calibration )?
- Did you use the right product, at the right dose?
- Did you follow the recommendations for use indicated on the packaging?
- Have you mixed incompatible products or too many products together?
- Did the applications take place under good conditions, particularly wind and temperature?
- Has manure with straw been brought into the crop?
- Are you sure of the quality of the irrigation water, could it not have been polluted by a herbicide in particular?
What to do ?
While there is no quick fix in this situation, you can take the following steps:
- clearly define the origin of phytotoxicity;
- prevent it from happening again;
- do not eliminate the plants immediately, lead them normally and observe their development; the latter 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, the type cultivated and the variety.
No other specific measure can be recommended.