G. C. M. Van Leeuwen

Yield Loss in Apple Caused by Monilinia fructigena (Aderh. & Ruhl.) Honey, and Spatio-temporal Dynamics of Disease Development

Monilinia fructigena (Aderh. & Ruhl.) Honey causes considerable yield losses in pome fruit culture. During a field study in the Netherlands in 1997 and 1998, the increase in disease incidence in time was assessed and final pre- and post-harvest losses were recorded in the susceptible apple cultivars James Grieve and Coxs Orange Pippin. Each individual tree was considered as a unique quadrat, and the spatial distribution of diseased fruits among fruit trees at every assessment date was characterised by a dispersion index, Lloyds index of patchiness (LIP). Spatial autocorrelation was applied to detect potential clustering of trees with diseased fruits within rows. In cv. James Grieve, the rate of increase of disease incidence was constant up to harvest time, whereas in cv. Coxs Orange Pippin disease incidence increased markedly 3 weeks before harvest time, which coincided with the harvest of cv. James Grieve in neighbouring rows. Pre-harvest disease incidence was 4.2–4.3% in cv. James Grieve in both years, in cv. Coxs Orange Pippin this was 4.4% in 1997 and 2.7% in 1998. Post-harvest yield losses amounted on average 1.5–2.0% for both cultivars, no significant differences were found between the cultivars (t-test, P=0.05). Both in 1997 and 1998, clustering of diseased fruits among fruit trees was detected; LIP values were significantly higher than 1 (P=0.05 in 1997, P=0.01 in 1998). Clustering of trees with diseased fruits was detected in 1998, when significant (P=0.05) positive correlation coefficients occurred for 2nd, 3rd and 4th lag-order distances in cv. James Grieve, and a significant (P=0.05) positive first-order correlation in cv. Coxs Orange Pippin. Wounding agents, such as insects and birds, may play an important role in the underlying disease dynamics, and crop losses may be minimised by control of these agents.

History of potato wart disease in Europe – a proposal for harmonisation in defining pathotypes

Potato wart disease, caused by the chytridiomycete Synchytrium endobioticum, was first introduced into Europe in the late 19th century. It spread quickly, and today is reported in 15 European countries. Initially, only one pathotype was found, and the disease was efficiently controlled using resistant cultivars. In 1941, however, formerly resistant cultivars showed wart formation in the field simultaneously in Germany and South Bohemia (Czech Republic), indicating the occurrence of new pathotypes. New pathotypes have since been reported from Germany, The Netherlands, Czech Republic, Ukraine and Canada. Today the pathogen is present in The Netherlands (only in fields for ware and starch potatoes) but restricted to two demarcated areas and subject to official control. Outside these areas, the pathogen is absent. For pathotyping, different countries have used different sets of differential cultivars, and the usual system of numerical coding of pathotypes has not been consistently followed. In this review we propose a new standardised code to be used for the 43 pathotypes currently known and described in Europe. The code is a combination of a numerical and letter code, combining the two terminologies used by former West and East Germany, respectively. We also plead for harmonisation in the choice of differential cultivars used for pathotype identification. The set of differentials described in the international standard for diagnosis of S. endobioticum issued by the European and Mediterranean Plant Protection Organisation (EPPO), should serve as a basis. Through close collaboration of European countries dealing with new pathotypes of potato wart disease, a final agreed upon set of differentials, combined with a set of reference isolates, should ultimately be established, allowing a clear distinction between the most important pathotypes occurring in Europe.

A novel technique using the Hendrickx centrifuge for extracting winter sporangia of Synchytrium endobioticum from soil

A zonal centrifugation method, known as the Hendrickx centrifuge technique, was tested for routine detection of winter sporangia of Synchytrium endobioticum in soil. In four experiments the ability of the Hendrickx centrifuge to extract the sporangia from soil was compared with a method used by the Dutch Plant Protection Service, which is a modification of the recommended EPPO method. Naturally and artificially contaminated soil samples were used to study the recovery percentage of and variation in numbers of winter sporangia. The effects of soil type and inoculum density were studied. The Hendrickx centrifuge method, developed originally for extraction of free living nematodes from soil, performed better than the method used by the Dutch Plant Protection Service. This was due to a better extraction recovery (60% higher), a lower measurement error (50% lower) and a lower detection level (down to 0.02 sporangia g−1 soil). The Hendrickx centrifuge method is much less labour-intensive than the method used by the Dutch Plant Protection Service. It can be used to extract many different organisms from soil, and DNA can be subsequently extracted from the supernatant for further PCR analysis. Inclusion of the Hendrickx centrifuge method in the official EPPO diagnostic protocol for regulated pests is recommended as an alternative method for detection of sporangia in soil.