Bacterial Diseases of Tomato and Strawberry
Pseudomonas syringae in Tomato (Lycopersicon esculentum Mill)
Pseudomonas syringae is a phytopathogenic bacterial species known as fluorescent because it produces yellow-green fluorescent pigments when grown in a nutrient medium with low iron content. This bacterium has a high capacity to spread. It is strictly aerobic, gram-negative, rod-shaped, and moves using a polar flagellum. P. syringae infection is characterized by the presence of necrotic spots surrounded by a chlorotic halo on the leaflets and, most importantly, severe stem blight characterized by necrotic cortical lesions without compromising the vascular tissue.
Direct Damage
Small black spots (1 to 2 mm in diameter) surrounded by a yellow halo form on the upper leaves. When numerous, these spots may coalesce and cause the leaflet to dry.
Control
- Avoid watering over the plant.
- Destroy diseased plants and fruits.
- Perform preventive treatments with copper.
Epidemiology
Wet and cool conditions favor disease development. Optimum temperatures for the disease are between 12-25°C. Bacteria tend to be present in seeds and disperse between plants via rain.
Survival
P. syringae can survive in crop residues, soil, seed, and alternate hosts throughout the year. It can withstand temperatures below 0°C and up to 51°C.
Erwinia carotovora
Taxonomy
- Order: Eubacterial
- Family: Enterobacteriaceae
- Genus: Erwinia
- Species: carotovora
Signs and Symptoms
E. carotovora causes soft rot and wet roots. In some cases, a foul odor may be present.
Direct Damage
Soft, damp rot occurs in the stem at different levels. Externally, blackish, wet areas are observed, and internally, the stem initially exhibits brown rot, darkening, softening, and emitting a foul smell.
Control Measures in the Field
- Avoid high soil moisture on the crown.
- Avoid severe mechanical damage to the roots during cultivation.
- Control other pathogens such as Fusarium, Phytophthora, nematodes, or insects.
Weather
Conditions favorable for disease development are high relative humidity and temperatures between 25 and 35°C, with the optimum being 22°C.
Transmission
E. carotovora overwinters in plant debris in the soil or the rhizosphere of non-host weeds.
Spread
The bacteria are spread by insects, tools, hands, rain, and irrigation water.
Penetration
Penetration generally occurs through injuries (frost, tillage, insects). Sometimes, in very moist soils, it can penetrate through healthy root lenticels.
Development
High humidity (above 90%) is required for the disease to progress successfully in its early stages. High temperatures accelerate its proliferation in intercellular spaces.
Prevention
- Avoid injury.
- Avoid excess moisture in the soil and on plant parts.
Treatment
Apply a systemic bactericide fungicide (insert specific product or mixture) to the soil, neck, and aerial parts periodically.
Angular Leaf Spot on Strawberry (Xanthomonas fragariae)
Name
Xanthomonas fragariae
Taxonomic Position
- Bacteria: Gracilicutes
Common Names
- Angular leaf spot (English)
Morphology
- Aerobic, Gram-negative, non-spore-forming, non-capped rod.
- Average size of 0.4 x 1.3 microns.
- Most cells are non-motile, but some have a single polar flagellum.
- Colonies are mainly circular, complete, convex, glossy, transparent to pale yellow.
- X. fragariae is a flagellated, rod-shaped bacterium, allowing it to move through an infected plant.
Symptoms
- Early symptoms begin as small, water-soaked lesions on the underside of the leaves.
- When humidity is high, lesions ooze sticky drops of bacteria.
- As the disease develops, lesions enlarge to form reddish-brown patches that later become necrotic.
Disease Cycle and Epidemiology
- Bacteria emanate from primary lesions, and bacterial cells are transmitted by aerosols caused by rain and sprinkler irrigation and transported by wind to healthy leaves.
- Penetration occurs through stomata. Infections of the crowns occur through wounds below affected leaves.
- The bacteria can attack flowers but not fruit. During epidemics, when environmental conditions favor exudation and diffusion, the bacteria can cause systemic infections associated with the crown.
- Systemic infections can arise in nurseries under humid conditions. Conditions that favor infection are moderate to cool daytime temperatures (around 20°C), low nighttime temperatures, and high humidity.
- Typical symptoms of the disease are lesions on leaves, fruits, and stems, and branch dieback.
Treatment
Copper fungicides have been recommended for controlling X. fragariae.
Xanthomonas campestris: Bacterial Scab
Xanthomonas campestris, used commercially to produce a high molecular weight polysaccharide that is an efficient viscosifier of aqueous solutions, causes black spots on all aerial parts of the plant, similar to P. syringae in tomato, but generally larger and more regular.
Symptoms
- The first symptoms on leaves appear as watery spots, darker green than the rest of the blade, circular to irregular.
- They become darker, turning brown with a black center on the adaxial surface, forming a small depression corresponding to a bulge on the underside.
- The center dries and takes on a shriveled appearance.
- Often causes spots on fruits, with a distinctive “bird’s eye” appearance.
- Crusted fruit.
Propagation
Penetration of leaves, cotyledons, and stems occurs through stomata, and fruits are infected through small wounds caused by wind, insect bites, etc. Rain, insects, and seeds are the main vehicles for spreading the disease.
Predisposing Conditions
Temperatures between 24 and 30°C and high humidity.
Morphology
- Occurs singly or in pairs, 0.6 x 1.0-1.5 microns, with a single polar flagellum.
- Sensitive to triphenyl tetrazolium chloride.