Symptoms resembling collar rot were detected in organically managed Topaz trees aged 3-10 years, occur-ring one to several years after planting of the orchard. Trees were killed within the same growing season in which symptoms were first observed. The disease commonly progressed as a complete covered canker at the base of the tree trunk. Isolation attempts were negative for Phytophthora and other Oomycetes, but con-sistently yielded Nectria galligena. The possibility of latent (endophytic) infections of N. galligena as the cause of delayed collar rot symptoms is briefly discussed

Klopp, Karsten

Weber, Roland W. S.

English

Organic Eprints

1 IBWF e.V., Erwin-Schroedinger-Str. 56, 67663 Kaiserslautern. 2 Obstbau-Versuchs- und Beratungszentrum Jork, Moorende 53, 21635 Jork. Nectria galligena as the cause of a collar rot disease in organically grown To-paz apple trees Nectria galligena als Ursache einer Kragenfäule-ähnlichen Krankheit bei der Apfelsor-te Topaz im ökologischen Obstbau Roland W. S. Weber 1 & Karsten Klopp 2 Abstract Symptoms resembling collar rot were detected in organically managed Topaz trees aged 3-10 years, occur-ring one to several years after planting of the orchard. Trees were killed within the same growing season in which symptoms were first observed. The disease commonly progressed as a complete covered canker at the base of the tree trunk. Isolation attempts were negative for Phytophthora and other Oomycetes, but con-sistently  yielded  Nectria  galligena.  The  possibility  of  latent  (endophytic)  infections  of N.  galligena  as  the cause of delayed collar rot symptoms is briefly discussed. Keywords: apple, canker, collar rot, endophyte, Nectria galligena, Topaz Introduction Seemingly typical collar rot symptoms were observed on apple trees (3-10 years old) grown in an organically managed apple orchard in Northern Germany’s most important fruit-growing area, the ‘Altes Land’ south-west of Hamburg. The trees were of the ‘Topaz’ variety grafted onto M9 root-stocks, with relatively low graft unions (10-20 cm above soil level). The disease commonly pro-gressed from the graft union upwards as a necrosis of the bark and underlying cambium tissue; upon removal of the bark, the advancing disease lesion was visible as a sharply delimited choco-late-brown zone on the underlying wood. No obvious root or crown rot symptoms were observed on the rootstock of the affected trees. Typical infections quickly enveloped the entire circumference of a tree, causing its death within one growing season. There was no clear-cut correlation with edaphic factors such as soil moisture, and infection foci were not observed. Instead, trees appeared to be affected randomly throughout the plantation.  The summary given here is based on a full account of our results provided by Weber et al. (2006).   Material and Methods With most of the above features pointing towards a Phytophthora collar rot, several attempts were made to isolate Oomycetes from the surface-sterilised bark of healthy and diseased trees, as well as their surrounding soil. A medium selective for Phytophthora and Pythium spp. was used, consist-ing of cornmeal agar (Difco) augmented with 50 mg nystatin (=mycostatin), 100 mg pentachloroni-trobenzene and 200 mg vancomycin litre-1 (modified from Erwin & Ribeiro, 1996). Our reference strain of Phytophthora cactorum as well as other Phytophthora and Pythium spp. showed good growth on this medium. Additionally, soil, root and bark samples were incubated in sterile distilled water  (Weber  et  al.,  2004).  Thirdly,  baits  (cut  apple  and  potato  halves)  were  buried  in  the rhizosphere of affected and healthy trees, recovered after 3 days, and plated out onto the selective medium as described above.  Results   Archived at http://orgprints.org/8840/     Apart from the sporadic baiting of Pythium sylvaticum from orchard soil, no Oomycetes were found in soil or plant tissues. Instead, one morphologically recognisable colony type belonging to a true (i.e. non-Oomycete) fungus dominated in our isolation experiments. This grew slowly on the selec-tive agar medium, but more rapidly on standard media without antibiotics. It was ubiquitous in bark samples taken from the advancing disease front, less frequent in dead tissue, and rare in tissue ahead of the advancing infection or in healthy trees (Fig. 1; Weber et al., 2006). Microscopic exami-nation and ITS/5.8 S rDNA sequence analysis permitted the unambiguous identification of this fun-gus as Nectria galligena (conidial state Cylindrocarpon heteronema).    Fig. 1. Agar plate with an Oomycete-selective medium showing the isolation of Nectria galligena as a slow-growing white colony from the advancing infection front (0) but not, in this case, from the healthy tis-sue 1 and 2 cm ahead of it (+1 and +2, respectively) or 1 cm behind it (-1).  Discussion Nectria galligena is the well-known cause of apple and pear canker. In its typical form the disease is initiated when conidia or ascospores germinate by emitting hyphae which penetrate through wounds or cracks into the bark of above-ground plant organs. Such infections result in open (partial) cankers in which callus-like outgrowths of bark tissue surround a central area of dead bark which often strips off to reveal the underlying wood (Fig. 2). Once such a canker has surrounded the entire circumfer-ence of a twig, all portions distal to it die back. Such a disease progression is usually very slow, tak-ing several years to kill the trunk of an established tree.  In the present case, disease symptoms were very different, with lesions frequently at the base of a tree, and disease progression being so rapid that the resulting canker was strikingly similar to a col-lar rot in entirely surrounding the tree trunk, in the dead bark remaining attached to the wood, and in lacking any callus-like growth. Infection with N. galligena therefore appeared to be systemic and endogenous, possibly arising from latent (endophytic) mycelium.  Endophytic fungi are capable of colonising living plant tissue for prolonged periods without causing disease symptoms. Fungi showing this form of association with terrestrial plants are known from   diverse taxonomic groups. Among those related to Nectria, the genus Fusarium is noteworthy in containing fully pathogenic species (e.g. F. oxysporum) as well as closely related strains which colo-nise their host tissue without overt disease symptoms (Kuldau & Yates, 2000). Other fungi may show a latent endophytic phase followed by an aggressive colonisation of the living plant (as patho-gens) or dead plant tissue (as saprotrophs). Could it be that N. galligena is capable, under certain circumstances, of establishing endophytic infections in its host, switching to its aggressively patho-genic phase later upon receiving appropriate environmental triggers? McCracken et al. (2003) have obtained some evidence for such a scenario; their study of genetic fingerprints of N. galligena iso-lates in apple orchards and nurseries showed that the inoculum of apple canker in young orchards was more closely related to that of the nurseries supplying the trees than to the natural inoculum on trees surrounding the orchard. In another recent study on N. galligena in organic management situations, Jansonius et al. (2004) have shown that there can be a very high incidence of apple canker in young trees especially during their first year in the orchard. This was explained by infection during the preceding autumn, i.e. after leaf fall but immediately prior to explanting into the orchard. The incidence of such delayed cankers was especially high in trees reared in organic nurseries. Although it is difficult to distinguish such a route of entry from latent endophytic infections, our observations of the rapid onset of complete and covered cankers at the base of trees several years after explantation into the orchard would point towards the existence of an endophytic phase, possibly after infection in the nursery as suggested by McCracken et al. (2003) and Jansonius et al. (2004).  We are currently examining the possibility of endophytism in N. galligena. If this feature turns out to be common, there will be obvious implications for the control of endophytic N. galligena infections in nurseries, especially in organic management practices where systemic fungicides cannot be used.   Fig. 2. Typical apple canker caused by Nectria galligena. The canker is partial and open, i.e. an area of bare wood is surrounded by callus-like host tissue. Literature Cited Erwin, D.C. & Ribeiro, O.K. (1996). Phytophthora Diseases Worldwide. APS Press: St. Paul, USA. Jansonius, P.J., de Jong, P.F. & Anbergen, R. (2004). Vruchtboomkanker in de biologische vruchtboomteelt. Publicatie LF79. Louis Bolk Instituut: Driebergen, Netherlands. Kuldau, G.A. & Yates, I.E. (2000). Evidence for Fusarium endophytes in cultivated and wild plants. In Micro-bial Endophytes (Bacon, C.W. & White, J.F., eds.), pp. 85-117. Marcel Dekker: New York, Basel. McCracken, A.R., Berrie, A., Barbara, D.J., Locke, T., Cooke, L.R., Phelps, K., Swinburne, T.R., Brown, A.E., Ellerker, B. & Langrell, S.R.H. (2003). Relative significance of nursery infections and orchard in-  oculum in the development and spread of apple canker (Nectria galligena) in young orchards. Plant Pathology 52: 553-566. Weber, R.W.S., Sulzer, F.-L. & Haarhaus, M. (2004). Pythium undulatum, cause of root rot of Abies procera Christmas trees and Pseudotsuga menziesii in Northern Germany. Mycological Progress 3: 179-188. Weber, R.W.S., Anke, H., Palm, G., Maxin, P. & Klopp, K. (2006). Absterbeerscheinungen bei Topaz-Apfelbäumen im Alten Land – Nectria oder Phytophthora? Mitteilungen des Obstbauversuchsringes des Alten Landes 61 (in press).  

Nectria galligena as the cause of a collar rot disease in organically grown Topaz apple trees

Abstract

Similar works

Full text

Available Versions

Organic Eprints