Aims Phase I

Interim Report Phase I (Hannover)
   - Thai-AMF isolation &       multiplication
   - AMF – nematode       interaction
   - AMF – nematode-       rhizobacteria interaction
   - AMF effect on nematode       feeding sites (histology)

Interim Report Phase I
(Bonn)

Aims and planned studies Phase II (Hannover)

Publications

Scientists:

Dr. H. v. Alten
C. Pech
P. Suwanarit

retired colleagues:
Dr. B. Masadeh
(finished her PhD)
R. Sikora

last update:
January 2008

The project will investigate the interrelationships between AMF (arbuscular my-corrhizal fungi) and microbes present in the root system of tomato in relation to bio-control of the root-knot nematode Meloidogyne incognita and plant health. Objectives are in the first Phase (year1-3): 1. To isolate and identify AMF, PHPR (plant health promoting rhizobacteria) and MHB (mycorrhiza helper bacteria) that control root-knot. 2. To differentiate interactions both in the rhizosphere and inside the root. 3. To explore synergistic interactions between AMF and microbes that increase biocontrol and root health 4. To examine the presence of inhibitory interactions with microbes. 5. To elucidate the mechanisms of biocontrol in single and concomitant systems. 6. To show the efficiency of combined inocula in a first experiment under practical conditions in Thailand. In the second Phase (year 4-6) we will try to develop formulated biocontrol inocula for tests in Thailand. Objectives: 1. Adaptability of biocontrol agents to local systems. 2. Interactions with other diseases e.g. bacterial wilt. 3. Integration of the biocontrol into farming systems in Thailand 4. Use of different application techniques. These aspects will be conducted together with partners in Thailand.


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Summary:

Improving biological control of Meloidogyne incognita by the synergism of mycorrhizae and rhizobacteria

Thai-AMF isolation & multiplication
	Trap cultures from 6 Thai soil samples Trap cultures from 8 AMF-inocula obtained from Prof. Poonpilai (Bangkok)
	Results:
		4 Thai AMF-isolates (free of Ralstonia) could be isolated and are now multiplied together with one tropical African isolate (36-366) for use in further experiments.
AMF – nematode interaction:
	AMF biocontrol effect on RK (mesh pot experiments): 3 AMF isolates were tested: 510, 49 and 36-366
	J2 attraction to mycorrhizal versus non mycorrhizal plants: Plants inoculated with 510 or not The experiment was repeated, additional isolate 36-366 was tested.
	Effects of root diffusates from mycorrhizal & non-mycorrizal plants on hatching of nematode eggs 2 AMF isolates were tested: 510 and 36-366.
	Results:
		Isolate 510 reduced, while 36-366 increased galls number compared to the control, isolate 49 had no effect. (The positive effect of 36-366 on gall number is due to a reduction of the root system)
		Galls sizes were significantly reduced by the use of isolates 510 and 36-366 but not by 49.
		Nematode juveniles were attracted to non-mycorrhizal plants. However, this is only relevant if the larvae have the chance to choose. We do not know at the moment whether this is relevant in a root system with mycorrhizal and non-mycorrhizal parts (the normal case).
		Root diffusates from mycorrhizal plants did not affect hatching levels compared with diffusates from non-mycorrhizal plants and/or water.
AMF – nematode- rhizobacteria interaction:
	Two rhizobacteria (SR1 and SR6) were tested against RK singly and jointly with the AMF isolate 510.
	Results:
		The rhizobacteria had no effect on number of galls when inoculated singly.
		510 could reduce number of nematode galls compared to the control, and this effect was doubled in joint inoculation with each of the two rhizobacteria tested. That means in our experimental system we found the “synergism” that we had put in our project title.
AMF effect on nematode feeding sites (histology):
	AMF-isolates 510 and 36-366 were used in 3 cultivars of different levels of RK susceptibility
	Results:
		One tomato cultivar not hosting the nematodes properly (tolerant 'Tiptop') showed normal size of giant cells. However, the typical alterations of the cell walls of transfer cells are developing slower in this cultivar.
		In cultivar 'Hildares' (highly sensitive to RK) AM-inoculation resulted in an enlargement of the giant-cell system. However, the number of nuclei in these cells was not increased. Cell wall alterations were not affected. In cultivar 'King Kong II' up to now no histological differences could be found after AMF-inoculation.

Summary:

Interrelationships between arbuscular mycorrhizal fungi (AMF) and rhizosphere bacteria in relation to the biocontrol of the root-knot nematode Meloidogyne incognita were investigated. Objectives in the first phase had been:

Fifty-five bacteria strains from the rhizosphere and mycorrhizosphere were isolated and identified. Two bacteria strains reduced the nematode penetration into tomato roots significantly. One isolate inhibited the in vitro growth of Rhizoctonia solani, Pythium ultimum and Sclerotinia sclerotiorum, but had no effect on the growth of Fusarium oxysporum. Five additional bacteria from the mycorrhizosphere recently isolated in Thailand and not yet identified have shown antagonistic activity against Pythium aphanidermatum in vitro. Six of 38 bacteria from the mycorrhizosphere had an effect on the growth of one aggressive strain Ralstonia solanacearum in vitro. Only one strain had a positive effect on mycorrhizal promotion. The cell wall components of Rhizobium etli G12 which are involved in induced systemic resistance against nematodes seem to play no role in the mycorrhiza promotion, whereas the treatment with viable bacteria was effective in increasing colonization. No systemic effect of Rhizobium etli G12 on mycorrhiza establishment was observed in split-root experiments.

The PHPR Rhizobium etli G12 promoted mycorrhiza establishment in the root of tomato in repeated experiments. The results open new opportunities for plant growth and health promotion, in that combinations of microorganism may have multiple effects on the plant and work synergistically. Our results demonstrated clearly that the promotion of AMF establishment was not due to systemic induced reactions in the plant. Cell wall components also can be excluded as a promoting factor in this interaction. Further studies are being conducted to test if there are effects of the bacterium on AMF spore germination.

Significant results were obtained in the experiments conducted with bacteria from the mycorrhizosphere on biocontrol activity against Meloidogyne incognita. Two of nine bacteria tested significantly reduced nematode penetration by more than 50%. This demonstrates that root colonized by AMF are also simultaneously colonized to a high degree by PHPR. Our studies show that AMF and rhizosphere bacteria build an antagonistic microbial community on or in the root system that increases root health. The combined application of AMF and RIB will be repeated in order to test the biocontrol management strategy based on the results of this studies.

Improving biological control of Meloidogyne incognita by the synergism of mycorrhizae and rhizobacteria

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