Tomato farmers often notice spots of various shapes and sizes on tomato fruits and leaves in the open field or inside greenhouses. These spots increase in size and number when the weather is humid and moderately warm. It is a sign that these plants need your help.
To find out how to keep tomatoes healthy, you need to identify the problem of the plant. Therefore, you need to scrutinize the spots on tomato leaves in your farm.
The first and most common possibility is that these spots are caused by increased exposure to sunlight (leaf scorch) and using pesticides or fertilizers at the wrong time. In this case, the spots will disappear on their own after a few days. If they don’t disappear or increase in number and size, it is time to take appropriate action and determine the cause so that we know how to treat it.
First, there’s no need to panic. Second, read the following text and start solving the problem.
The main cause of spots on tomato fruits and leaves is bacteria. There are at least seven types of bacteria that cause spotting, the most important of which are:
Bacterial canker of tomato (Clavibacter michiganensis subsp. michiganensis)
Pathogen: Bacteria responsible for bacterial canker which causes white spots on leaflets, with symptoms similar to a bird’s eye. This disease causes much larger damage of a different nature.
This bacterium enters through the stomata where the initial infection starts and then extends to the surrounding tissues.
Symptoms of the disease:
-Edges of leaf turn brown.
-Vascular discoloration, yellow and then brown, and the presence of a sticky fluid.
-White spots with brown centers on the fruit (bird-eye spot).
Family: Nightshades: Tomatoes, eggplant, capsicum, and potatoes.
Conditions suitable for disease spread: Moderately warm temperature and high humidity, fast-growing tomatoes are infected more than slow-growing tomatoes.
This disease is widely spread in Turkey, Iran, and the United States.
Transmission conditions: Mainly seed-borne.
Economic importance: Losses range from 50% to 90% of the crop if the disease is not controlled early.
-The use of healthy seeds is the first and most important condition for disease control.
-It’s preferred to use seeds that have been soaked in an acid solution (such as sulfuric acid) or Novo Safe.
-Significant reduction in infection can be achieved by chemical seed treatment.
-Once the disease appears in the crop, stringent health measures, such as destroying infected plants and isolating infected rows, can reduce crop yield loss.
-Preventive measures (destruction of residues from the previous crop, sterilization and disinfection of greenhouses and equipment) are necessary to prevent protected crops from becoming infected.
Resistant varieties are available but have not yet been largely incorporated into commercial varieties.
Bacterial speck (Pseudomonas syringae pv. Tomato)
Pseudomonas syringae pv. tomato causes small, irregular, brown spots/specks 1-2 mm in diameter. These spots are gradually surrounded by a yellow halo.
If conditions are particularly wet, they expand, fuse together, and affect whole sectors of the leaf. Brown elongated lesions may develop on petioles and flowers initially, causing the latter to fall off. The fruits clearly present tiny black spots with a green aura.
Bacterial speck affects all parts of the plant and needs high relative humidity and
temperature of more than 20°C for epidemic spread.
Bacterial spot (Xanthomonas campestris pv. Vesicatoria)
The pathogen is a bacterium that survives on plant debris and weeds, and it is airborne, seed-borne, and waterborne. It infects the plants through wounds on leaves and fruit and appears in the form of black, circular water spots on the leaves. These spots soon wither and fall, affecting the stem and fruits.
Symptoms of infection
-Symptoms of bacterial infection appear on leaves, stems, and fruits. Fruit infections are the most harmful.
-Initially, the symptoms appear on the leaves in the form of small, greasy yellow spots with a diameter of no more than 3 millimeters. As the disease progresses, the spots become angular and turn dark brown or black. The center of the spots dries then and falls off.
-Similar spots appear on stems and petioles, but they are elongated. Cankers may form on the affected parts of the old stems.
-This bacterium infects fruits only when they are young and green, but symptoms continue to appear throughout the various stages of fruit’s growth.
-The fruit spots – in the beginning – are very small and black, and they may be surrounded by a small white aura which disappears later. As the infection progresses, the spots enlarge until they are about 5 mm in diameter and become brown. They are slightly low and have a scabby appearance. Infected fruits may show skin cracking and become vulnerable to infection by other mold-causing organisms.
Spread and appropriate conditions for infection:
-Bacterial spot and bacterial speck in tomatoes are spread in hot and humid weather, especially when the foliage is wet with dew, rain, or irrigation.
-This bacterium lives in plant debris in the soil, and infection occurs through contact and wounds.
To control the disease, the following methods are recommended:
- Follow a long agricultural cycle.
- Use disease-free seeds and seedlings.
- Dispose of infected plants outside the field.
- Spray with copper-based compounds.
- Plant resistant varieties.
- Cover planting rows with plastic covers to prevent plants from being exposed to rain in regions where significant rainfall may occur.
-Since this bacterium is seed-borne, disease-free seeds and seedlings must be used. Usually, the seeds are soaked in a sulfuric acid solution (0,5m) for 3 minutes or in Novo Safe
-To prevent disease in the field when spots appear, copper-based compounds are sprayed (1kg/200 L of water) like Samaya, Oxy-copper, Hydro-copper, or Scudo. Spraying is repeated 3 times at intervals of 15 days.
Bacterial canker (Pseudomonas syringae pv. syringae)
Pseudomonas syringae pv. syringae causes small brownish to black leaf spots fairly comparable to those of bacterial speck. A yellow halo sometimes surrounds these spots. This bacterium is also associated with leaf necrosis, particularly that localized at the periphery of the leaf.
Types of Xanthomonas spp.
Several species of Xanthomonas have recently been associated with bacterial spots: X. euvesicatoria, X. vesicatoria, X. perforans, and X. gardneri.
These bacteria cause small, moist spots at first, soon becoming brown to black and not exceeding 2-3 mm in diameter. They also feature a yellow halo, which is slightly more discreet.
Some elongated moist, brown symptoms are also visible on petioles and stems. The lesions on the fruit are very characteristic and translucent at first, and then become carcinogenic gradually.
Similar symptoms can be caused by hail damage.
Prevention and Control of all Bacterial Diseases/Tomato
- Use disease-free seeds from trusted sources, and healthy seedlings. Do not purchase any seedlings that present spots on leaves or stems.
- Plant resistant varieties as much as possible.
- Make compost from the remnants of the previous crop and sterilize the greenhouse and its walls. Do not plant in soil that is known to be infected by the previous crop.
- The agricultural cycle, especially in the event of a severe infection in the field. Do not plant nightshades in a field that was heavily infected the previous season.
- Control weeds, especially broadleaf ones.
- Provide good air movement, avoid moist conditions, and adopt good soil drainage plan such as watering in the early morning.
- Avoid high-pressure sprays, as these may injure leaves enough to encourage the introduction of the bacterial pathogen.
- Avoid working in plants when wet. Do not harvest crops and do not let workers into the field when there is dew.
- Clean hands and use clean and disinfected tools. Do not allow workers harvest a field of nightshades and then move to another field without changing their clothes and washing their hands with soap and water before starting work.
Chemical control during the season:
Use Novo Treat when planting, when the conditions of infection apply, and when the symptoms appear, at a rate of 2L/200L water in foliar treatment. Spraying to be repeated several times during the season. Do not spray while, before, or after using copper-based or sulfur-based compounds.
As a result of high temperature and humidity, these symptoms of bacterial spots have spread:
by W. Al Matni