Coniella diplodiella (Speg.) Petr. & Syd., the causal organism of white rot of vine is presently regarded as an organism of quarantine importance to South Africa on all grapevine propagating material as well as fresh fruit imported to South Africa.

Not much is known about this disease locally, and the aim of the present note is therefore to briefly summarise the known literature on this topic, and to place this into perspective with the situation in South Africa.

Disease symptoms and importance

The pathogen typically infects berry clusters before veraison. Infection is usually associated with hail damage, as the fungus needs wounds through which to infect. That said, however, rain-splash carrying clods of soil or stones may also initiate infection. The pathogen requires temperatures above 15C for infection, and conidia (fungal propagules) germinate and infect rapidly at 24-27C. Infected berries lose their turgidity and become densely covered by pycnidia (fungal fruit bodies). Pycnidia raise the cuticle without rupturing it. Air that penetrates the area between the cuticle and epidermis make berries appear white. Mature pycnidia also have a white outer crusty layer of cells, and thus the name “white rot” has been coined for this disease.

Under favourable conditions of high temperature and humidity the fungus can spread systemically from infected berries through the pedicel to the rachis. C. diplodiella can also cause cankers on non-lignified shoots. Infections are most prevalent around the nodes of green shoots, and appear as long depressed, brownish, necrotic areas. At the end of the season infected berries fall to the ground, eventually releasing their pycnidia and conidia into the soil.

The pathogen has two stages in its life cycle, namely a short phase while infecting the berry, and a longer dormant phase in vineyard soil. White rot affected vineyards have been found to contain between 300-2000 conidia per gram of soil. Pycnidia can still release viable conidia after more than 15 years, clearly stressing the importance of keeping this pathogen out of vineyards not previously contaminated. Vineyards in which the pathogen is present will have to be handled differently to keep all shoots as high as possible from the soil, to hopefully avoid all rain-splash of soil particles and subsequent infection. Losses of between 20-80% have been recorded in Europe, underlining the fact that under certain conditions C. diplodiella can be a serious pathogen of grapevines (Bisiach, 1988).

Geographic distribution

It is presently known that C. diplodiella is widely distributed internationally, and is closely associated with its host. It has been reported from most countries in Europe, from North and South America, Australasia, Oceania, Asia and Africa. In the latter continent it has been reported from South Africa (Verbeek, 1977; Matthee & Thomas, 1981), Tanzania and Zambia (IMI distribution map No. 335, ed. 3, issued 1992).

The first record of this fungus occurring in South Africa can be found in an Annual Report of the Secretary for Agricultural Services (Verbeek, 1977), and on this basis it has also been included in the list of Phytopathogenic Fungi from South Africa (Crous et al., 2000). A second report of this disease was published by Matthee & Thomas (1981). In neither of these reports was the morphology of the fungus dealt with, nor any reference herbarium material lodged in the National Collection of Fungi in Pretoria. It has therefore not clearly been proven that this pathogen occurs in South Africa.

Present status of Coniella on grapevines

Once we started investigating white rot of grapevines, we discovered that in addition to C. diplodiella, several other species of Coniella in fact occur on this host world-wide. These species are also known to have wide host ranges (Sutton, 1980). Furthermore, although not on grapevine, some of these species are known to occur in South Africa. Of relevance to vines are species such as C. petrakii B. Sutton, C. fragariae (Oudem.) B. Sutton (Crous et al., 2000) and C. granati (Sacc.) Petrak & Syd. (IMI 233050). In inoculation studies on grapevine, isolates of C. petrakii, and to a lesser extent C. fragariae, have also been shown to cause white rot symptoms (Tiedemann, 1985). Isolates of a Coniella sp. have recently been obtained from roots of grapevines in South Africa, and we are presently determining the identification and if this species can cause white rot of vines.

Global movement of pathogens

Given the enhanced import and export of agricultural products, it becomes the duty of National Plant Protection Organisations world-wide to protect their agricultural and forestry industries from harmful, exotic pathogens entering that country. For pathogens to be placed on an actionable list and be reckoned as of quarantine importance to a country it must comply with the international definition for a quarantine organism. A quarantine pest/organism is a pest/organism of potential economic importance to the area endangered thereby and not yet present there, or present but not widely distributed and being officially controlled. The listing of pathogens/pests as of quarantine importance is based on a risk analysis, relevant documentation and discussions with the industries and plant pathologists. In terms of the WTO SPS agreements countries are permitted to stop or impede imports because of the occurrence of quarantine organisms. Such decisions are, however, based on sound scientific evidence. Incorrect South African literature reports have in the past resulted in problems encountered with the export of agriculture fresh produce. For this reason, the Southern African Society for Plant Pathology has determined that all local fungal plant disease reports must be supported by herbarium specimens and, wherever possible, fungal cultures.

The occurrence of a specific pathogen in a country does not justify that products infected with that pathogen can now be freely imported. The reason for this is that each pathogen population consists of numerous genotypes or races that differ significantly in their ability to cause disease. The great potato famine caused by Phytophthora infestans (Mont.) de Bary in 1845-1847 in Ireland, resulted in more than a million deaths, and forced millions more Irish to emigrate from Ireland. This pathogen has subsequently spread world-wide with its host. Recently, however, a new mating type has emerged, and is slowly being dispersed (mainly via export) to countries where it was formerly absent. The introduction of opposing mating types enables fungi to undergo sexual recombination, creating numerous new races that can overcome all local resistant cultivars. Only strict phytosanitary control can prevent such mating types or races form entering South Africa, and thus indirectly protect our local industries. Coniella diplodiella has reportedly also been observed to have a teleomorph (sexual state), though little is known about its occurrence. Could it be that the correct mating types are still not widely distributed If C. diplodiella does indeed occur in South Africa, it does not presently appear to be a serious disease. Could the reason for this be that the original genotypes introduced to South Africa were less virulent than those common in Europe It is now also apparent that more than one species of Coniella can cause white rot of grapevines (Tiedemann, 1985), and that they have wide host ranges (Sutton, 1980), which further complicates matters surrounding this disease. Based on reported cases in literature and the devastating consequences that such new introductions have had on various industries world-wide, it is the opinion that no further contributions be made to our grapevine disease problems. It is therefore proposed that white rot of grapevines, caused by imported species of Coniella, be retained as a disease of quarantine significance until local grapevine isolates have been compared on a molecular basis with the established international gene pool of these fungi.

References

Bisiach, M. 1998. White rot. In: Compendium of Grape Diseases (eds E.C. Pearson & A.C. Goheen), pp. 22-23. APS Press, St. Paul, Minnesota, USA.

Crous, P.W., Phillips, A.J.L. & Baxter, A.P. 2000. Phytopathogenic fungi from South Africa. University of Stellenbosch Printers: In press.

Matthee, F.N. & Thomas, A.C. 1981. Rot Blanc in vines: a new disease in South Africa. Deciduous Fruit Grower 31: 268-273.

Sutton, B.C. 1980. The Coelomycetes. Fungi Imperfecti with pycnidia, acervuli and stromata. Kew, Surrey, England.

Tiedemann, A. von. 1985. Untersuchungen zur pathogenitt des erregers der weissfule (Coniella petrakii Sutt.) an Amerikaner- und Europerreben und zur verbreitung und bedeutung des pilzes in den Deutschen weinbaugebieten. Ph.D. Dissertation, Georg-August University, Gttingen, Germany.

Verbeek, W.A. 1977. Plant Pathology. In: Annual Report of the Secretary for Agricultural Technical Services for the period 1 July 1975 30 June 1976. Pretoria: Government Printer.

The Authors:

Pedro W. Crous: Department of Plant Pathology, University of Stellenbosch, P. Bag X1, Matieland 7602
Elma Carstens: National Department of Agriculture, Directorate Plant Health and Quality, Private Bag X5015, Stellenbosch, 7599

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