BACTERIAL WITHER OF TOMATOES – Clavibacter michiganensis
Symptoms of the disease (characteristics for the diagnosis of the disease)
Disease Overview: This highly damaging disease affects tomatoes in both greenhouse and field settings. Its symptomatology is complex, primarily manifesting in the aerial parts of the plants, while the root system often shows no distinctive symptoms.
Types of Symptoms: Two main categories of symptoms are identified based on the affected tissues:
- Vascular Symptoms: These symptoms include wilting of the plants, necrosis of the vascular tissues, and the presence of open wounds (ulcerations or “cankers”).
- Local Symptoms: These encompass leaf scorch, “bird's eye” spots on fruit, and lesions surrounding the peduncle cavity.
Dominant Symptoms: The most prominent symptom observed in naturally infected tomato crops in greenhouses is plant wilting. This symptom develops gradually during the flowering period as soil temperatures rise above 20ºC. Wilting typically starts in the lower leaves, characterized by twisting leaflets that bend to one side, creating a concave shape toward the affected leaflets. This phase lasts a short time, after which rapid, widespread wilting occurs throughout the plant.
Tissue Observations: When the stem or leaf axis is sectioned in areas showing wilting, a slight yellowing of the vascular tissues is noticeable. Over time, the conducting vessels in the affected organs may turn yellow-brown or completely brown, while the medullary tissue may become macerated, changing color from yellow to reddish-brown.
Effects of High Humidity: In conditions of high atmospheric humidity (above 85%), elongated blackish-gray streaks can appear on the stems, petioles, and leaf veins in protected environments. These streaks indicate tissue disorganization and cracking, leading to open wounds (ulcerations or "cancers") where the bacterium can easily exude and spread within the crop.
Leaf Burning: Another significant symptom related to the frequency of infection is leaf burning, characterized by “scorched” spots of vibrant green tissue rich in bacterial mass.
Systemic Infections: In cases of systemic infection, flowers often abort, while those that remain tend to produce infected fruits that develop slowly. These fruits may be small, deformed, ripen prematurely, easily detach from the inflorescence, and have brown seeds.
External Infections: Particularly in field-grown tomato crops, "bird's eye" spots are observed on the fruit. These appear as white spots that eventually turn brown, either remaining lenticular or rounded with a white or yellowish halo. This symptom is characteristic of the presence of this bacterium.
Factors favoring the occurrence and evolution of the disease, its transmission and spread
- Sources of Infection and Transmission: Seeds and soil are the primary sources of bacterial infection, facilitating the transmission of pathogens from one growing season to the next. The bacterium can spread through various maintenance activities, including mulching, defoliation, fleshing, extraradicular fertilization, and even foliar applications of insecticides, which can carry the pathogen from infected areas (infection foci) to healthy ones, thus increasing the affected area. Additionally, precipitation combined with wind plays a significant role in the dispersal of bacteria within crops.
- Observations on Nitrogen Fertilization: Our long-term observations indicate that nitrogen fertilization—especially when not preceded by agrochemical analysis to justify its necessity—can lead to excessive and lush vegetative growth. This condition may inadvertently encourage the onset and progression of bacterial wilt in tomatoes.
- Prevention and Control Measures: Effective prevention and control of this pathogen are challenging and require strict adherence to a comprehensive set of measures, including:
- Soil Disinfection: Chemical disinfection of soil in greenhouses using Metam sodium (Nemasol 510) at a rate of 500 liters per hectare, following the proper application protocol. This treatment should be applied in areas where monoculture has been practiced for over 5-6 years and where plants infected with this pathogen or other soilborne pathogens (fungi and bacteria) have been observed.
- Seed Disinfection: It is crucial to recognize that seeds can be a primary source of infection. When establishing a tomato crop in a greenhouse for the first time and observing signs of Clavibacter michiganensis subsp. michiganensis, it is likely that some seeds were carriers of the bacterium. Two methods are available for disinfecting tomato seeds: a) Thermal disinfection by soaking seeds in warm water at 53ºC for 1 hour. b) Chemical disinfection through vacuum infiltration.
- Since these effective seed disinfection methods can currently only be performed domestically, we recommend the following strategies against this pathogen:
- Cultural Practices: Recognizing that the first disease symptoms typically appear 4-5 weeks after planting (when soil temperatures exceed 20ºC), nursery activities should be delayed. Continuous monitoring of the tomato crop in greenhouses should occur for 4-5 clear days, observing plant turgor. Daily phytosanitary inspections should be conducted to check for wilting symptoms on the basal leaves. If any plants show these symptoms and do not recover after 1-3 days, they should be removed from the crop to prevent the spread of bacteria; the vacated space can then be filled by neighboring plants.
- Ventilation: Ensure proper ventilation in greenhouses to reduce humidity.
- Condensation Prevention: Take measures to prevent condensation on crop leaves within greenhouses.
- Balanced Fertilization: Implement balanced nitrogen fertilization strategies.
- Local Infection Management: To limit local infections in tomato crops—especially following any green work that causes plant injuries or in fields exhibiting “bird’s eye” spot symptoms—foliar treatments should be administered using copper-based products such as Funguran OH 300 SC (0.15% Copper Hydroxide) or Bouillie Bordelaise (0.75% Metallic Copper Sulfate).
- Root Application: Consider applying AXEROOT (0.35%), a growth stimulator with a balanced formulation of phytohormones, amino acids, phosphorus, folic acid, and other rooting cofactors.
