EVIDENCE FOR THE PATHOGENITY OF ACTINOMYCETES IN ROOTLETS OF APPLE SEEDLINGS FROM SOILS CONDUCIVE TO SPECIFIC APPLE REPLANT DISEASE

Light microscopic investigations of rootlets of young apple seedlings grown in soils known to suffer from specific apple replant disease (SARD), consistently yielded colonization of the epidermal and cortical tissue by actinomycetes.
In some cases, fungal hyphae and nematodes were also observed in the rootlets in addition to actinomycetes.
Although the fraction of rootlets containing actinomycetes was much higher than those of rootlets exhibiting mixed colonization.
These results suggest that the infection of rootlets by the actinomycetes is a primary one.
The actinomycetes were able to penetrate the cortical tissue via epidermal cells and via root hairs to cause root rot.

Histological examination of epidermal and cortical tissue of apple rootlets from a SARD soil by transmission electron microscopy revealed the occurrence of branched hyphae.
Due to the characteristics of these hyphae they were considered to be actinomycetes.
The hyphae were observed as early as 28 days and the frequency of their occurrence increased with advanced age of the plants.

Large scale anomalies of the cell wall were observed within the cells of the infected rootlets.
In addition to these anomalies, further modifications of the cell wall/plasmalemma interface occurred.
Growth of the hyphae predominated in the intercellular spaces.
Their ability to penetrate through the cell walls into the cell lumen was demonstrated.
The mode of activity of the hyphae in the cellular tissue and the reactions induced by them showed that the observed organism is a pathogen.

IV International Symposium on Replant Problems

K. Szabó, H. Winkler, H. Petzold, R. Marwitz

https://doi.org/10.17660/ActaHortic.1998.477.6