|Ph.D Student||Tardi-Ovadia Rinat|
|Subject||Development of a Method for Detection and Iidentification|
of the Phyto-Pathogens Helminthosporium Solani and
Colletotrichum Coccodes on Solanum
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Raphael Linker|
|Dr. Leah Zror|
|Full Thesis text|
Potato (Solanum tuberosum) is a major crop in terms of total food production, fourth after rice, wheat and corn. Blemish disease downgrade the economic value of potatoes and may cause penalties or rejection of the potatoes for processing. Therefore, it is important to minimize the development of these diseases in the field as well as during storage.
Symptoms of silver scurf caused by Helminthosporium solani and black dot caused by Colletotrichum coccodes respectively are frequently confused due to the similarity of blemishes caused on the periderm. At first silver scurf lesions are light brown and circular which may become a dark olive color, the symptoms enlarge during storage and silver discoloration of the periderm appears. Both fungi may be present simultaneously on the same tuber. Therefore, in order to minimize the economic loss, it is important to differentiate between them in early stages of disease development. To date there is no commercial, accurate and low cost method available.
Fungi are known to modify pH locally as part of the inoculation and establishment process, and the first objective of the present study was to develop a method enabling the measurement of pH modification at tissue level during artificial inoculation in situ. Such pH monitoring may provide valuable information on host-fungus interactions. Direct visualization and estimation of pH change near the inoculation area were achieved using pH indicators and image analysis using an RGB channel calibration. The pH in the infected area shifted in more than 1.4 units from the potato native average pH. It is the first report on the ability of H. solani to cause alkalization of the infected area. An interesting phenomenon was observed during the first 17 days after inoculation: a halo appeared at the margin of the lesion formed after inoculation with H. solani.
The second part of the study focused on the investigation of fungus metabolic profile in different growth conditions. Fungus metabolic profile was evaluated by Biolog FF microplates containing 95 distinct carbon sources. The quantification of substrate utilization dynamics was achieved by fitting a mathematical model to the observed growth/utilization curves. The model allowed us to derive two key kinetic parameters for each fungus: lag phase, and maximal growth rate. Analysis revealed substrates in which significant differences between C. coccodes and H. solani were observed. These substrates may further assist us in the design of growth media enabling differentiation between the two fungi. A similar analysis comparing the metabolic profiles of sub-cultured fungus and fungus culture from lesion revealed significant differences. It was previously known that sub-culturing causes a decline in fungal virulence; we may assume that the virulence decline can be associated with change in the fungus metabolic profile.
We hope that better understanding of fungus-host interaction at the tissue level as well as the identification of differences in metabolic profile will ultimately assist in the development of efficient method for detection, identification and management of C. coccodes and H. solani.