Fig. 1 Brown-black vascular streaking typical of Petri disease.

Paul Fourie¹ and Pedro Crous^2
1Disease Management, ARC Infruitec-Nietvoorbij, Stellenbosch
²Department of Plant Pathology, University of Stellenbosch

Trying to name the fungus...

In 1912 an Italian plant pathologist, Petri, found species of Cephalosporium and Acremonium to be associated with brown wood streaking in grapevine (See Fig. 1 above) [1]. A few years later in California, Chiarappa [2] demonstrated that the Cephalosporium sp. was in fact the main causal organism of this disease. Hawksworth et al. (1976) referred to this fungus as being similar to Phialophora parasitica, a fungus associated with various disease conditions of woody hosts, and infections in humans [3].

Fig. 2 Conidiophores, conidia and chlamydospore-like structures of Phaeomoniella chlamydopora on potato dextrose agar.

Trying to name the disease...

Various names were used for this disease, ranging from black goo to slow dieback, slow decline and Phaeoacremonium grapevine decline [8]. During a recent International Workshop on Grapevine Trunk Diseases held in Portugal (September 2001), consensus was reached that the disease will henceforth be called Petri disease.

Trying to understand the disease...

Although Petri disease is caused by Pch, several Phaeoacremonium spp. have also been implicated in this disease [8]. These pathogens are also seen as the precursors to the esca disease complex (Fig 3).

Fig. 3 Esca wood decay symptoms in 27-year-old vine.

Esca is regarded as the most important disease of older grapevines in Europe [8]. Repeated isolation of Pch and several Phaeoacremonium spp. from asymptomatic, young grafted vines, rootstock cuttings and grapevine trunks suggest that these fungi can also occur as endophytes. Conidia, which are formed inside the host, are dispersed in xylem sap. The major means of spread appears therefore to be through infected propagation material. However, Pch also produces a phoma-like synanamorph on canes and litter [6], resulting in inoculum dispersal via rain splash and irrigation, leading to pruning wound infection [9]. Within the host Pch grows in or around the xylem vessels (Fig. 4), producing pectinolytic enzymes and secondary metabolites that have been implicated in foliar symptom expression. Stress situations in vineyards enhance disease.

Fig. 4 Mycelium growth in xylem vessels (SEM photo by Lizél Mostert).

External symptoms include chlorotic foliage with necrotic margins, stunted shoot growth, retarded sprouting, absence of sprouting, wilting and dieback. Internal wood symptoms range from brown to black vascular streaking, visible in cross sections as spots or circular discolouration of the xylem tissue (Petri disease, Fig 1). A tar-like substance can sometimes be seen oozing from the severed vessels. This discolouration was attributed to the host's reaction to the presence of the pathogen. Tyloses and phenolic compounds block xylem vessels, which eventually lead to an insufficient water and nutrient supply and thus slow dieback (Fig 3). Vascular streaking can furthermore lead to dark brown stains in the wood bordering on a creamy soft rot in the pith of aging vines (esca, Fig 5).

Fig. 5 Decline and dieback of 2-year-old grapevines.

Although molecular kits have been developed to detect the pathogens in apparently healthy grapevine material [10, 11], little is presently understood about the infection process, and synergism between the various pathogens in this disease complex. Future work is focusing on understanding the epidemiology and the evaluation of various means of disease control. Petri disease is presently regarded as one of the most serious diseases affecting young vineyards worldwide.

Literature

1. Petri, L. 1912. Osservazioni sopra le alterazioni del legno della vite in seguito a ferite. Le Stazioni Sperimentali Agrarie Italiane 45: 501-547.
2. Chiarappa, L. 1959. Wood decay of the grapevine and its relationship with black measles disease. Phytopathology 49: 510-519.
3. Hawksworth, D.L., Gibson, I.A.S. & Gams, W. 1976. Phialophora parasitica associated with disease conditions in various trees. Transactions of the British Mycological Society 66: 427-431.
4. Ferreira, J.H.S., van Wyk, P.S. & Venter, E. 1994. Slow dieback of grapevine: association of Phialophora parasitica with slow dieback of grapevines. South African Journal of Enology and Viticulture 15: 9-11.
5. Crous, P.W., Gams, W., Wingfield, M.J. & van Wyk, P.S. 1996. Phaeoacremonium gen. nov. associated with wilt and decline diseases of woody hosts and human infections. Mycologia 88: 786-796.
6. Crous, P.W. & Gams, W. 1999. Phaeomoniella chlamydospora gen. et. comb. nov., the causal organism of Petri grapevine decline and esca. Phytopathologia Mediterranea 39: 112-118.
7. Groenewald, M., Kang, J.C., Crous, P.W. & Gams, W. 2001. ITS and ß-tubulin phylogeny of Phaeoacremonium and Phaeomoniella spp. Mycological Research 105: 651-657.
8. Mugnai, L., Graniti, A. & Surico, G. 1999. Esca (black measles) and brown wood-streaking: two old and elusive diseases of grapevines. Plant Disease 83: 404-414.
9. Larignon, P. & Dubos, B. 2000. Preliminary studies on the biology of Phaeoacremonium. Phytopathologia Mediterranea 39: 184-189.
10. Groenewald, M., Bellstedt, D.U. & Crous, P.W. 2000. A PCR-based method for the detection of Phaeomoniella chlamydospora in grapevines. South African Journal of Science 96: 43-46.
11. Tegli, S., Bertelli, E. & Surico, G. 2000. Sequence analysis of ITS ribosomal DNA in five Phaeoacremonium species and development of a PCR-based assay for the detection of P. chlamydosporum and P. aleophilum in grapevine tissue. Phytopathologia Mediterranea 39: 134-149.