Fastidious diseases in grapevines – towards a friendly solution

December 1, 2020
Fastidious diseases in grapevines – towards a friendly solution

Yellows disease and esca phenomenon are two fastidious diseases of grapevines. Yellows disease is caused by several types of Candidatus phytoplasma (class Mollicute) – walless bacteria which reside either in the host plant phloem or in the guts of the insect vector. Esca phenomenon is a complex of at least three fungal species which reside in the xylem and pith of the host plant. Although both diseases are considered as very serious and cause severe losses to the growers, currently there is no solution at hand to control them. The complex etiology of each of the diseases poses many constraints for research: The pathogen of the yellows disease is an obligatory parasite which cannot be grown in an artificial medium. Artificial infection can be done only via an insect vector, grafting or a dodder plant. Symptoms on the vines can be detected only in the summer but the vine in many cases remains infected forever. The pathogen is distributed unevenly in the vines’ phloem tissue, therefore, not all the canes and clusters exhibit the specific symptoms.

 

The pathogens associated with the esca phenomenon establish in the vine’s trunk very slowly (years), before foliar symptoms are visible. Foliar symptoms are also unevenly distributed, their appearance is linked to environmental variables and they are inconsistent year to year. The complexity of both pathogens required the development of an adequate experimental system for each case. Regarding the study of yellows disease, experimental systems using Spiroplasma melliferum as a model organism and nurse culture for phytoplasma of grapevine plantlets were developed. Regarding the study of esca- we adopted common in vitro and in planta methods and adapted them to our conditions. Specific primers to identify the local strains of the pathogens were constructed, consequently saving a lot of time and labor.

 

 

Recently the role of endophytes1 in plant defense was suggested by many studies. Plants and non-pathogenic microorganisms are associated by various forms from mutual to symbiotic interactions. Where mutual associations occur, the host plant protects its endophytic inhabitants from biotic and abiotic stresses. On the other hand, endophytes benefit their hosts by various protective mechanisms and/or by promoting plant growth.The endophytic population within the vines changes between healthy and infected vines. Furthermore, spontaneous recovery from yellows disease is a known phenomenon in grapevines.

 

 

The symptoms in recovered vines disappear and the vines regain their normal growth and productivity within 2-3 years, suggesting a role for endophytes. However, in order to look for a potential biocontrol agent (BCA), the candidate must be cultivable. Several commercial products based on beneficial micro-organisms are currently available as biological control agents against other plant pathogens. In search for a friendly solution to protect the vines from yellows disease and esca phenomenon, we isolated endophytic bacteria from vines and from the insect vector of the yellows pathogen. These endophytes which were screened for their inhibitory activity, resulted in two bacterium species that show promising traits as potential BCAs.

The specific experimental systems enabled us the discovery of Frateuria defendens (Frd), a novel bacterium isolated from the guts of Hyalesthes obsoletus, which is the prevalent vector of stolbur2-type phytoplasma. Frd showed inhibitory activity not only in model systems but also under field conditions.

 

 

Although isolated from the insect gut, Frd can penetrate various plant species from different families like Vitaceae, Rutaceae, Cucurbitaceae, Solanaceae and Apocynaceae. Cells of Frd inhabit the vascular system and survive in planta up to several weeks. A field trial during two consecutive years was conducted where Frd was applied to severely infected vines. The application of Frd had halted the increase of the infection rate in treated vines, compared to untreated vines. Thus, infection rate of treated vines had increased from 41% in the first year to 46% in the second year compared to an increase from 43% to 58% in untreated vines.

 

   

 

Furthermore, the yield of infected vines treated with Frd, had increased by 17% and recovery rate by 6.5%, compared to untreated vines. Regarding the esca phenomenon, a beneficial bacterium (named E4), that was isolated from vines, showed an inhibitory activity not only in vitro but also in semi field trials. In vitro tests show 90-99% inhibition of mycelium growth of the three pathogens in the presence of E4 compared to control. In planta experiments showed a difference in pathogen response to E4 virulence. We found that out of the three main pathogens associated with esca, the most aggressive pathogen is the most sensitive to E4 treatment. Preliminary studies show that the mode of action of both beneficial endophytes involves the secretion of secondary substances that prevent the growth of phytoplasma and esca pathogens.

 

*Vered Naor – Shamir Research Institute

 

The work is done in collaboration with Dr. Einat Zchori-Fein- and Dr. Ofir Bahar (ARO), Dr. LilachIasur-Kruh (Braude College) and Dr. Tirtza Zahavi (Extension Services, Israeli Ministry of Agriculture). 1.endophytes are non pathogenic micro-organisms that reside within the plant. 2.stolbur-type phytoplasma is the pathogenic type found in Israeli vineyards.  

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