Immunological characteristics of the collection material of cucumber for resistance to downy mildew in greenhouse conditions

Abstract

The most harmful and economically significant disease affecting cucumber in greenhouses is downy mildew, the causative agent is Pseudoperonospora cubensis Rostw. The priority direction of the world's selection of this vegetable culture is the creation and introduction into the production of varieties and hybrids with a set of features that combine high productivity and resistance to downy mildew. For successful breeding work on the creation of resistant to downy mildew cucumber hybrids important comprehensive study of the immunological potential of collection specimens. It is known that the resistance of plants to pathogens often depends on their ability to react at the early stages of infection, with the formation of necrosis of tissues, mainly caused by quinones ( oxidized phenols). The rate of oxidation of phenols is due to the activity of enzymes, in particular polyphenol oxidase and peroxidase. The peculiarity of oxidizing systems in resistance forms is that the infection causes the activation of oxidative processes, whereas in susceptible ones - inhibition or absence of activation.

The purpose of the research – to carry out an immunological evaluation of the collecting material of the parthenocarpic cucumber gherkin type for resistance to downy mildew.

Materials and methods of research. The research was conducted during 2016-2018 in film greenhouses of the Laboratory of Selection of Cucurbits of the Institute of Vegetable and Melon Growing of HAAS. Experiments are carried out in accordance with "Methodology of experimental work in vegetable and melon-growing". Studies to evaluate the stability of specimens against downy mildew  were carried out in accordance with "Phytopathological evaluation of a vegetable  breeding material."

Collection specimens were evaluated on a rigid natural infectious background (permanent greenhouse soil plus monoculture). Collection specimens included 26 specimens from: the Netherlands, Russia, the Czech Republic, Germany, Turkey, South Korea, Ukraine. The level of resistance to downy mildew was estimated in the phase of the beginning of mass fruiting plants. The level of redox enzymes was studied in laboratory conditions using the Boyarkin method in the leaves of cucumber of the upper layer in the phases of the beginning (without visual signs of defeat) and and mass fruiting  (in leaves with symptoms of downy mildew). For analysis, two specimens from each resistance group were taken. The sample for each specimens was 10 plants.

Research results and their discussion. The highest resistance specimens (score 9 of the immunological scale) were not found among collections of specimens. It was found that 4 specimens were highly resistant to this disease under the conditions of the natural provocative infectious background: Golubchik F₁, Zircon F₁, Chinese climbing F1 and Dutch F₁. Immunologic reaction to the damage of the downy mildew in these specimens was characterized by small local spots on the leafy plates. The affected areas did not merge together, their number grew moderately and gradually.

Medium resistance (score 5) was recorded in 8 specimens: Olga F₁, Zelenyiy potok F₁, Solon F₁, Parker F₁, Pasalimo F₁, Shakti F₁, Alligator F₁, Marinda F₁. In this group of specimens, symptoms were manifested as yellow, rounded spots of 2-3 mm in size. First, they appeared on the edges of physiologically aging leaves, and then gradually spread over the entire area of the leaf surface.

Low resistance (score 3 ) had 10 collection specimens: Athena F₁, Anuschka F₁, Lenara F₁, Kibriya F₁, Arctic F₁, Artist F₁, Kuzya F₁, Sedrik F₁, Daphne F₁, Eliza F₁. This group of plants signs of damage of downy mildew  first  was shown on the upper side of leaf in a form of a solid yellowish angular spots. They were sharply limited by the leaf veins, eventually lost the contrast, blending together. On the underside of the leaves where the lesions the causative agent, formed angular weeping spot on which was formed the gray-brown layer of  sporulation.

Very low resistance (score 1) had 4 specimens: № 11, Capricorn F₁, Miranda F₁, Hrustyaschaya gryadka F₁. It is on these specimens that the disease manifested itself in the form of yellow, large sizes (12 mm spots), limited by leaf veins. After 7-10 days, the spots merged the entire leaf turned yellow, and a few days later the affected tissue was dried.

Along with the determination of the specific manifestation of the immunological reaction in different resistance groups of cucumber, the level of activity of polifenoloxidase and peroxidase enzymes in the control (healthy plants) and experimental (affected by the pathogen) specimens was analyzed.

The increasing of peroxidase activity in the affected leaves in comparison with healthy ones by 240-290% was observed in highly resistant specimens. In specimens with medium resistance, this parameter increased by 155-190 %. A similar picture was found in infected plant tissues susceptible specimens. However, the growth of peroxidase activity in comparison with healthy leaves was less intense – 67-130 %.

Analysis of laboratory data revealed that the level of activity of peroxidase in healthy cucumber leaves varies depending on the resistance group and ranges from 22.73 to 25.00 mg-eq. / g s in very low-resistance, 33.78 - 36.46 mg-eq. / g s in highly resistant specimens.  Peroxidase activity in healthy leaves specimens Golubchik F₁ and  Zircon F₁ was 1.21 - 1.47 times higher than in medium resistant specimens – Zelenyiy potok F₁ and Olga F₁ and very low resistance specimens № 11 and Capricorn F₁.

There was no significant difference in polyphenol oxidase activity in cucumber healthy leaves by groups of resistance, however, in the affected leaves of high resistant samples, this parameter increased by 363–388%, in medium resistant ones by 100–220%, in samples with low and very low resistance - by 30 –74%.

 It has been proved that the activity of redox enzymes in cucumber leaves can be an additional diagnostic specific parameter in the selection of valuable sources for the selection of parthenocarpic type cucumbers on  downy mildew resistance.