Diseases of fruit plantation medicinal and aromatic crops (2+1)

Angular leaf spot

Causal organism: Xanthomonas fragariae.
The disease was first reported from Minnesota in 1960, and it appears to be spreading rapidly to many strawberry-growing areas of the world with the import of planting materials.
Symptoms

• Typical symptoms of angular leaf spot appear initially as minute, water-soaked lesions on the lower leaf surface (Figure).
• These lesions enlarge to become angular spots, usually delineated by small veins.
• An important distinguishing characteristic of this disease is that lesions are translucent when viewed in transmitted light, but dark green when viewed in reflected light (Figure).
• Under moist conditions, lesions often have a viscous bacterial exudate on the lower leaf surface. The exudate forms a whitish, scaly film when dry.
• This exudate or film is an additional characteristic useful for the identification of angular leaf spot disease.
• Lesions may coalesce to cover large portions of the leaf.
• Eventually, lesions become visible on the upper leaf surface as irregular, reddish-brown spots which are necrotic and opaque in transmitted light.
• A chlorotic halo may surround the lesion.
• At this stage, it is difficult to distinguish the symptoms from those of common leaf spot and leaf scorch disease.
• Heavily infected leaves may die, especially if major veins are infected.
• Occasionally, under natural conditions, infection follows the major veins, resulting in veinal water-soaking that may or may not spread to the interveinal regions.
• Infection by X. fragariae may become systemic.
• The pathogen can infect all the plant parts except fruits but roots and, in some cases, even the fruits have been infected, apparently only in the tissue adjacent to an infected calyx (fruit cap).
• Calyx infection can be serious. Infected tissues turn black resulting in unattractive fruit development
Disease Development

• Inoculum for the primary infection of new growth in the spring comes from infected dead leaves in which the pathogen overwinters.
• X. fragariae may survive for extended periods in dry leaves or in infected leaves buried in the soil. Spread is primarily from infected leaf debris or infected crowns.
• Bacteria that exude from lesions under high-moisture conditions may provide secondary inoculum.
• Bacteria may be disseminated to uninfected plants or through splashing air water or overhead irrigation.
• X. fragariae gains entrance into host tissue either passively through injuries or actively as motile cells that swim into natural plant openings by means of drops of dew, gutation fluid, rain or irrigation water.



Epidemiology

• Disease is found by moderate to cool daytime temperature around 200 C,
• low night temperature (near or just below freezing),
• high relative humidity (92- 97%),
• long periods of precipitation,
• Sprinkler irrigation to protect plants from freezing or heavy dews in the spring also favor the disease.
• Young leaf tissue or leaves on healthy, vigorous plants are more likely to become infected than those on diseased or environmentally stressed plants.

Management

• Identification of the disease and removal of runners at the early stage reduce the disease
• Picking and destroying old blooms and overwintered canes help in reducing the disease.
• Avoiding excess irrigation helps to check the disease.
• Fungicidal spray like Copper oxy chloride (0.2%) is effective in its control.
• The disease is effectively controlled by spraying streptocycline (0.05%) .