|Phomopsis (dead arm) is an economically important disease in grapevines and occurs in most vine producing regions of the world.
In Australia, four Phomopsis species have been associated with this disease. The current study found three of these species in South African vineyards, as well as a new species, Phomopsis amygdali, which traditionally causes a die-back disease in peach trees.
Dead arm in vines is caused by Phomopsis viticola. Isolations from vines with typical die-back symptoms produce this pathogen together with Botryosphaeria dothidea (Phillips, 1998) and Eutypa armeniacea (Dye & Carter, 1976). However, the dead arm symptom is more generally associated with Eutypa armeniacea, which causes eutypa die-back (Moller & Kasimatis, 1981). For this reason the general name Phomopsis is preferred to that of dead arm. Typical symptoms include brown, round lesions surrounded by pale yellow areas on the leaves (Figure 1) and brown to black lens-shaped lesions on the first three to four internodes of green shoots (Figure 2). The disease can result in crop loss due to shoots breaking off near the basis where the lesions formed, reduced growth in vine shoots, loss of vigour, smaller bunches and sometimes as a result of fruit being infected. Crop losses of up to 50% have been reported (Pine, 1958). Actual economic losses are usually only found in older vineyard blocks due to the slow dying back of the vine.
Two new Phomopsis species, P. longiparaphysata (Uecker & Ker-Chung, 1992) and P. vitimegaspora (Ker-Chung & Lii-Sin, 1998) were recently isolated from vines in Taiwan. In a further study in Australia, four additional Phomopsis species were found on vines (Merrin et al., 1995). The purpose of this study was therefore to determine which Phomopsis species occur on South African vines, and the implications for disease control.
The true identity of the organism causing Phomopsis was investigated on plant material collected from 58 different vineyards in the vine producing areas of the Western Cape. P. viticola was isolated from diseased vineyard material from Lutzville to Swellendam, but not from the Oudtshoorn and Orange River vine producing areas. Figure 3 indicates the distribution of Phomopsis in the sample area. Phomopsis is widespread in the Western Cape and according to Marais (1981), serious incidences of Phomopsis have occurred in the Helderberg, Firgrove, Somerset West, Rawsonville and Slanghoek producing areas.
Phomopsis species in South African vineyards
Diaporthe perjuncta (sexual stage of Phomopsis species 1), P. viticola, P. amygdali and an unidentified Phomopsis species occur on vines in South Africa. P. amygdali is associated with shoot scorching of peach trees in the U.S.A. Similar symptoms also occur on peach trees in S.A. and it is therefore quite possible that this pathogen jumped from peaches to vines. Further research is required, however, before this can be confirmed. Species differentiation is based on morphological (Figure 4) and cultural (Figure 5) characteristics, stem inoculations (Figure 6) and the formation of the teleomorph in culture. The identity of each species was furthermore confirmed by means of phylogenetic analyses of the nuclear ribosomal DNS internal transcribing spacers (ITS1 and ITS2) and the 5′ point partial nucleotid sequence of the mitochondrial small subunit (mtSSU) (Mostert et al., 2000b).
Phomopsis amygdali was isolated once only from vines and currently seems to be less important in this disease complex. Diaporthe perjuncta and the unknown Phomopsis species are relatively uncommon. Judging from the stem inoculations on green shoots, these species do not seem to be pathogenic. However, Brand et al. (1999) found that budding is inhibited by D. perjuncta, which was often isolated from eyes that did not bud. P. viticola, on the other hand, was commonly isolated and associated with typical disease symptoms. This species is obviously the main pathogen of this complex. Isolations from symptom free material showed that P. viticola occurs in the lower nodes and internodes of one-year-old canes, thus indicating that the fungus hibernates in these plant sections (Mostert et al., 2000a). What is more, the molecular data indicates that the unknown Phomopsis species and P. amygdali are not host specific on vines. However, P. amygdali caused a pathogenic reaction on peaches as well as vines.
Disease control depends on disease pressure which varies from year to year, depending on the remaining inoculum and the climate. Phomopsis can be controlled effectively by integrating sanitation practices with applications of fungicide. Shoots with typical lesions and dead wood must be removed when pruning, since these lesions provide most of the inoculum for new infections. For effective control, the well-considered application of fungicides in the vegetative stages of young shoots is important. In South Africa the following fungicides have been registered to control Phomopsis: copper oxychloride, copper oxychloride/sulphur, copper sulphate/lime, folpet, phosethyl-A1 + mankozeb, propineb and sulphur (Nel et al., 1999). Of these folpet, phosethyl-A1 + mankozeb and mankozeb are used most frequently (Swart et al., 1994). The new strobilurin fungicides have not been registered for Phomopsis in South Africa. These new fungicides have various advantages. Strobilurin fungicides are active against a wide range of organisms, are quasi systemic, are easily absorbed by the plant, are environmentally friendly and display no contra-indications with fungicides that are currently on the market (Ypema & Gold, 1999). Three of the strobilirin formulations were tested in the laboratory for effectiveness in inhibiting spore germination and mycelium growth. These fungicides’ base line sensitivity was determined towards P. viticola. These fungicides provided good inhibition of spore germination and mycelium growth at low concentrations. Further trials are necessary, however, to test the effectiveness of these fungicides in the field. Strobilirin fungicides are currently used to control powdery mildew in vineyards. The application hereof in early spring should also have an inhibiting effect on Phomopsis.
In South Africa pre-budding control of Phomopsis does not occur. Fungicides registered for this time bracket in other countries are not user friendly and the application thereof is labour intensive. In vines where Phomopsis poses a serious problem, the control of inoculum on the previous season’s pruned shoots will reduce the occurrence of the disease. The development and testing of pre-budding fungicides is an area which warrants further research.
Various Phomopsis species occur on vines in South Africa. P. viticola is the Phomopsis species causing disease and control programmes should focus on this organism. P. viticola hibernates in the lower nodes and internodes of shoots. The use of strobilirin appears to be a good alternative should the disease pressure of P. viticola increase, although further field trials will be required to confirm this. Of the other three Phomopsis species that were isolated, only P. amygdali is pathogenic on vines.
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Lizél Mostert & Pedro Crous
Plant Pathology Department, Stellenbosch University, Private Bag X1, Matieland 7602