3.7.1
BACTERIAL DISEASES OF CONIFER SEEDLINGS AND THEIR CONTROL IN THE FOREST NURSERIES OF SIBERIA

ID GRODNITSKAIA

Sukachev Institute of Forest Research, Akademgorodok, Krasnoyarsk 660036, Russia

Background and objectives
Seedlings in Siberian forest nurseries are damaged by infectious diseases: necroses and chloroses of needles, wilting due to injuries of roots, tumors and sores on stems. The diseases are caused by bacteria and micromycetes. Unlike the fungi, phytopathogenic bacteria are poorly studied, even more so in Central Siberia. Chemicals used to control these microorganisms often suppress populations of non-phytopathogenic microbes. Their niches become vacant and can be occupied by potentially harmful microbial species that progressively acquire pathogenic properties. To avoid such by-side effects, biological methods of protection of seedlings should be developed, which are less aggressive to the environment and do not disturb the fragile stability of agricultural ecosystems. The main goal of this research was to identify phytopathogenic bacteria damaging seedling in nurseries and to find such microbes that can live in the conditions of nurseries, exhibiting antagonistic activity in relation to the pathogens.

Materials and methods
Damaged and dead sprouts and 1-4 year old seedlings of Pinus sibirica, P. sylvestris, and Larix sibirica were studied for the presence of different bacterial strains, which were cultivated on agar-containing media and next tested for phytopathogenic and antagonistic properties. The most active antagonists were checked for their ability to suppress the growth of phytopathogens in open air conditions. The effectiveness of treatment of seeds by suspension of the antagonistic microbes was assessed judging from the difference in the numbers of viable seedlings in the treated and control samples.

Results and conclusions
Sprouts and the most young seedlings do not contain the amounts of nutrients sufficient to effectively defend themselves from pathogens by means of intensification of phenolic metabolism [1, 2]. For this reason, sprouts are most vulnerable to diseases, and the highest death rate was found within the first year after sowing. Thus, mortality of seeds equaled 36%, while that of sprouts was 24%. Survey of the reasons of damage to juvenile conifers was conducted in Central Siberia for the first time. It revealed that Pseudomonas fluorescens Migula caused blackening of proximal parts of needles and terminal buds, damaging 15% of 1 year old seedlings and bringing them to death. Bacterial infection of vascular bundles of 2 and 3 year old pine seedlings was caused by Pseudomonas solanacearum Smiht. Different strains of Pseudomonas syringae van Hall with accompanying epiphytic species Erwinia herbicola provoked bacterial blister in 3 year old pine seedlings and cancerous gnarls on the stems of 4 year old pines and larches. Decaying of roots and stem bases was caused in pine seedlings by Bacillus mycoides Flugge and Bac. mesentericus Flugge, which normally are saprotrophic soil bacteria in natural ecosystems. Agrobacterium radiobacter and A. tumefaciens gave rise to tumors on root collars in both pine and larch seedlings. Bacteria Xanthomonas spp. were isolated from drying shoots of 2-4 year old larches and pines; Xanthomonas ampelina Panagopoulos caused chloroses and necroses of needles. Hence, infection was promoted not only by typical pathogens, but also by saprotrophs which live in soil in the absence of conditions suitable for parasitism. Laboratory tests also indicated microoganisms with pronounced antagonism to the listed phytopathogenic bacteria. The antagonists belonged to specific strains of Bacillus subtilis, Pseudomonas fluorescens, Ps. aureofaciens. The seeds of Pinus sylvestris treated before sowing by water suspension of these strains gave 16-36% more sprouts and 42-83% more viable seedlings of bigger size in the end of the first summer. The seeds treated by the same suspensions before storage were stored 3-4 times more safely. Therefore, usage of microbial antagonists can effectively increase output of healthy conifer seedlings in forest nurseries.

References
1. Friend J, 1979. Recent Adv. Phytochem. 12, 557-588.
2. Mace ME, 1989. Wilt Diseases. Berlin, 163-174.