Causes and prevention methods for leaf necrosis in lettuce

This type of damage is one of the main causes of yield reduction in lettuce. Browning develops along the edges of the outer leaves or on the heart leaves, and necrosis sometimes appears on the veins. The affected tissue zones serve as ideal entry points for various pathogens, in particular, the agents of gray mold and soft bacterial rot.

Bacteria Pseudomonas marginalis and P. cichorii have been isolated from necrotic leaf tissues. These are secondary pathogens that colonize dead tissues, but when they reach healthy tissues, they can cause them to decay.

Necrosis on lettuce leaves can develop under the influence of various environmental conditions. In all cases, the appearance of symptoms is closely related to disturbances in the plant water regime.

Active water loss by the plant leads to dehydration of leaf tissues, especially at the edges, where cells gradually die. Dehydration is caused by low relative humidity and high temperatures. Under such conditions, with simultaneous impairment of water uptake by the root system, plant damage is almost inevitable. A shallow or poorly functioning root system, dry soil, low soil temperature, and high salt content in the soil are all factors that limit water uptake by the roots. On mature lettuce plants, the damage develops much faster, which is associated with a larger total leaf surface area and, consequently, greater water loss. As a rule, when maintaining soil moisture at the required level and preventing excessive air temperature fluctuations in the greenhouse, such leaf damage is not observed.

SYMPTOMS, CAUSES, AND CONTROL MEASURES FOR LETTUCE TIPBURN Leaf water loss excessive | insufficient General name Tipburn Glassiness Marginal scorch Vein scorch Withering Marginal spotting of leaves Symptoms Necrosis along plate edges, Marginal necrosis of plates, usually particularly severe on interveinal, especially on young aging leaves leaves Cause Leaf water release is stronger Root water uptake is stronger than than root uptake leaf release Control 1. Proper soil water 1. Maintain moderate relative air regime humidity for long periods 2. Development of a healthy 2. Uniform ventilation over the root system crop for more active water 3. Maintain low concentration evaporation by leaves of soluble salts in the soil If water uptake by the plant is more active than its release, similar symptoms develop. With the normal functioning of the root system in warm, moist soil and high relative humidity, the activity of water release through the leaves weakens sharply, and plants with a well-developed root system are more easily affected by tipburn. Reducing air humidity and active ventilation helps to prevent leaf damage by tipburn. In the early stages of development, it can sometimes be detected by the appearance of the leaves, the edges of which become watery and glassy. If measures are taken immediately to reduce humidity in this case, necrosis does not develop. Herbicide damage Contact herbicides cause necrosis and death of lettuce plants, while growth regulator-type preparations in sublethal doses cause plant deformation, with symptoms of damage by various herbicides being similar in the latter case. Within 24 hours after their application, plant leaves usually curl downward, and the development of new leaves stops depending on the dose applied. Very often, leaf plates decrease in size, and in extreme cases, the main vein curls at the end, giving the leaf a typical appearance (the so-called "dog tongue" symptom). This symptom я а и: Г р а ь | \.й ’- + 9 ь {< я 0: „#% \Ф. 1 { м Зы» о ах хеатоь м, ®. т мм. я ый”, А “2” Я зы Fig. 9.6. Symptom of leaf damage by a sublethal dose of MCPA. appears under the influence of sublethal doses of MCPA, 2,4-D, and 2,3,6-TBA. Plant stems can elongate, and adventitious roots form, especially near the soil surface. Under the influence of 2,3,6-TBA, leaf edges fuse, forming "cups" or "pitchers." When applying sublethal doses of MCPA (mecoprop) and dichlorprop, leaf edges curl downward, some changes in interveinal tissues occur, stems can elongate, and adventitious roots form at the nodes.

With soil application of most growth regulator type herbicides, small nodules form on the roots. Damage due to high soil acidity

Lettuce grown on acidic soils (usually pH from 6 to 5.5 for peat soils) lags in growth, and leaves sometimes turn red. In extreme conditions, roots become dull gray and very small. Adjusting soil acidity with lime makes corrections for this, although this technique is recommended to be carried out at a pH no higher than 6.5.

Excessive application of inorganic fertilizer can lead to an accumulation of soluble salts in the soil in very high concentrations. This hinders water uptake by plant roots. Lettuce is very sensitive to such conditions; its growth is stunted, and it often forms false heads. Soil conductivity within 2600 mmhos (index 3) is considered the maximum for the lettuce crop. Before planting, it is necessary to determine the salt content in the soil.

Low-temperature injury Symptoms of low-temperature injury include uneven leaf growth, which becomes curled or crinkled (see Fig. 9.1). Sometimes tissues on the outer leaves begin to peel off, air bubbles accumulate underneath, and the affected leaves appear silvery. Mature leaves sometimes turn red. FURTHER READING Sisiterg$, ГеНисе, Мтог Уеве Без апа Мизйгоот$. Мпигу о! АрпА|пулск, Мо{питБегапа. Gaze, J. E. (1972) Glasshouse Lettuce. Grower Books, London. Morgan, W. and Tiffin, A. I. (1979) Pest and Disease Control in Glasshouse Crops. British Crop Protection Council Publications, Croydon. CHAPTER 10. PEPPER AND EGGPLANT