Influence of bacteria resistance to cold on the resurgence of strawberry angular leaf spot

The causal agent of bacterial angular spot in strawberry plants, the bacterium Xanthomonas fragariae is viable even under intense cold. This explains its presence in leaves damaged by frost, left in crops under the sun

19.01.2018 | 21:59 (UTC -3)

Bacteria are microscopic, unicellular organisms that have a cell wall and can be phytopathogenic. Within the genus of bacteria xanthomonas, more than 100 species that infect plants have been identified. The species Xanthomonas fragariae Kennedy & King, 1962, is the causal agent of bacterial angular leaf spot on strawberry plants. It is a potentially serious disease, which can cause losses of up to 75% of fruits and was first reported in the United States and later described in New Zealand, Australia, some Asian and African countries and in most European countries where strawberries are grown. . X. fragariae is easily transmitted through asymptomatic plants with latent infections that are exported and this may have led to the introduction of X. fragariae in the USA and to many other countries.

This species has the ability to remain viable even at low temperatures. Due to the cold autumn climate in the Mexican highlands where the work was carried out, frost-damaged leaves are generally left in crops under the ground, serving as primary inoculum for the next spring crop.

Symptoms of the disease are small, soggy angular spots (1mm-4mm) that initially appear only on the underside of the leaves and surrounded by veins. In the initial phase, the spots are only visible on the lower surface and appear translucent when viewed with transmitted light. The bacteria are spread from the spots by rain, irrigation or dew and begin new infections, often along the main veins of the leaf. With high relative humidity, white, milky, cream or yellow exudate may appear. Leaves are most sensitive when they are two weeks to two months old, older and younger leaves are resistant to infection. The size of the lesions progressively increases and, subsequently, the spots may coalesce and become apparent on the upper surface of the leaf. Necrosis is characterized as an irregular reddish-brown spot, which, when worn, can rupture the leaf blade.

This test sought to investigate the presence of X. fragariae viable in frost-injured leaves collected in a field infested by this bacterium.

An area was chosen for collection whose reports of bacterial diseases always referred to the bacteria of interest in this assay. 20 samples were collected in the field, including borders and the center of the one-hectare area. Leaves damaged by frost, left under the ground, were collected, cleaned with an air jet and macerated in a mortar with distilled water. The suspension obtained was diluted in sterile water and, with the aid of subculture loops, was used to inoculate petri dishes with nutrient agar medium, and were then incubated at 28°C for 96 hours. After the appearance of the colonies, they were purified and the young colonies obtained were subjected to tests of glucose metabolism, starch degradation, media with 0,01% and 0,1% triphenyl tetrazolium chloride, production of hydrogen sulfide, hypersensitivity test in smoke and rot in potatoes, with the aim of determining the presence of X. fragariae. The tests were all done in duplicate.

Among the 20 samples analyzed, 16 expressed the expected result for X. fragaria, demonstrating that this bacterium remains infective even in tissues damaged by frost.

It is interesting to note that of all the tests carried out, only the smoke hypersensitivity test is capable of differentiating X. fragariae de X. arboricola p.v. fragariae, also present in Mexico and a common occurrence, responsible for the burning of strawberry plants.

The bacteria was detected in most of the samples analyzed, confirming the importance of leaves damaged by frost as a source of disease inoculum for the next strawberry production cycles.

 Click here to read the article in Cultivar Vegetables and Fruits in issue 78. 


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