Marginal necrosis

(tipburn)

Under the name marginal necrosis are grouped together two types of abiotic problems (physiological or non-parasitic) affecting lettuces grown in the soil, as above ground.

Marginal necrosis of the leaves bordering the apple and of the heart

( internal marginal necrosis , internal tipburn ) (figures 1 to 8)

• Symptoms  :
  • development of wet necroses on young leaves (figure 1);
  • translucent appearance of the small veins located on the periphery of the leaf blade which take on a brown hue (figure 2);
  • diffuse browning of the surrounding tissues and installation of a soft rot;
  • presence sometimes in places of tiny spots and browning of short portions of veins.

To observe this problem on salads with “closed” heads, it is necessary to cut them. It manifests itself more as the salads approach maturity. At this stage, the leaves of the heart protected by those of the crown, and therefore less airy, are more humid.

• Cause(s) : Calcium in salads: it is a component of cell walls that acts as a binder of contiguous cell walls. This element is also involved in cell division and elongation. It is passively absorbed by young roots and transported via xylem vessels. Once inside the tissues it is not mobile. For it to be able to migrate sufficiently in the lettuces towards the growing tissues, it is essential that these transpire. In the end, the lack of foliar calcium is not due to too little of this element in the roots, but to several environmental factors. Under these conditions, damage is caused by the collapse of membranes and cell walls.

• Diagnostic clues  :
  • ambient humidity too high;
  • insufficient plant transpiration;
  • limited shelter ventilation;
  • unbalanced nitrogen fertilizer;
  • prolonged period of overcast, foggy weather;
  • limited root development;
  • too rapid growth of young leaves exposed to strong daytime light intensities, etc.

• Remedies* :
  • perform a soil analysis before planting to ensure balanced manure and avoid excess nitrogen and calcium deficiencies;
  • control the calcium concentration of your nutrient solution
  • avoid cultivating sensitive varieties, sometimes showing a limited root system;
  • promote plant transpiration;
  • ventilate the shelters as much as possible (opening of openings, fans);
  • limit or even stop irrigation during periods of high humidity;
  • promote the proper functioning of plants as much as possible;
  • controlling plant growth, preventing it from being too rapid;
  • avoid excessive accent lighting;
  • controlling carbon dioxide (CO2) enrichment;
  • counteract sudden rises in temperature with light drench baths, etc.

* When the problem appeared, no measure made it possible to remedy the symptoms that appeared on the lettuces.

Marginal necrosis of crown leaves

(dry or tipburn marginal necrosis) (Figures 9-11)

• Symptoms  :
  • development of more or less extensive brown to black lesions appearing at the edge of the blades of rather low leaves (figures 3 and 4);
  • nearby small veins may also turn brown;
  • thereafter, these rather localized lesions spread rapidly;
  • eventually, brown patches, which look rather dry, gradually reach the inside of the limbus;
  • more or less marked yellowing of the limbus on the margin of the lesions;
  • We could associate it with "latex necrosis" which also results in a dry alteration of the periphery of the limbus, but which is preceded by bursting of the vessels and the local emission of this milky compound.

• Cause(s) : The causes of this problem seem less well defined; it would be partly linked to an accumulation of salts on the periphery of the blade, where the veins end.

•  Diagnostic clues :
  • a reduced supply of water to the leaves, leading to a reduction in the flow of calcium and/or by a rupture of the laticiferous ducts releasing toxic latex;
  • insufficient plant water supply;
  • excessive ventilation;
  • too rapid evolution of climatic conditions towards heat and drought;
  • poor condition of the root system (poor establishment, damage linked to biotic or abiotic attacks – excess salinity);
  • inappropriate nitrogen fertilizer and/or excessive evaporation due to wind or excessive ventilation under shelter, etc.

• Remedies :
  
  • perform a soil analysis before planting to ensure manure and avoid excess nitrogen and calcium deficiencies;
  • prepare the soil well and promote good rooting of the plants;
  • choose a variety with a vigorous root system that is often less sensitive;
  • ensure an water supply, especially during dry climatic periods;
  • above ground, avoid EC (electro-conductivity) that is too high;
  • control the salinity of the nutrient solution , the supply of water and the ventilation of the shelters in particular when the hygrometry is low, etc.

Note that tipburn could also appear after harvest, during storage, and even in consumers. In addition to depreciating the commercial quality of lettuces, these various necroses constitute excellent nutritive bases from which opportunistic microorganisms, such as Choanephora cucumerina , Botrytis cinerea (figure 12), various bacteria, settle. They lead to leaf and apple rot, especially if the ambient humidity is high. It should also be noted that lettuce, lettuce in particular, although moderately sensitive to salinity, may show similar symptoms following sudden increases in the concentration of soil salts.