22 trapman.rtf

Archived at http://orgprints.org/8841/

Observations on the epidemiology of Sooty Blotch in Organic orchards in the Netherlands Beobachtungen zur Epidemiologe der Regenfleckenkrankheit Marc Trapman Abstract From 2002 to 2005 field observations where made on the occurrence of Sooty Blotch (SB) in unsprayed orchards on the apple variety Topaz. First SB symptoms in high inoculum orchards where found after on average 639 leaf wetness hours following the first rain 10 days after petal fall. Even then symptoms where present only on places on the fruits that probably stayed wet longer than the leaf wetness sensors have indicated. Weather stations that are set up to register average orchard conditions are unlikely to detect the wetness conditions that lead to SB infections in critical parts of the orchard. From the delayed symptom development compared to the published data form USA we concluded that under Dutch conditions the lower temperature retards the symptom development. In the first month after the first symptoms where found disease progress was slow, but increased towards the harvest. The gradual increase of both disease incidence and severity, and the appearance of the symptoms suggest ongoing new infections on the fruits during summer rather than one major SB infection that spreads on the fruit. Within a year the disease incidence between orchards varied form 0 to over 90%, but between years the disease incidence in an individual orchard stayed in the same range. This makes disease history an important factor in the panning of a control strategy.

Keywords: Sooty Blotch, epidemiology, control Introduction In fruit growing regions in Europe where climatic conditions are very favorable for apple scab infections organic fruit growers plant modern Vf scab resistant varieties like Topaz, Red Topaz and Santana. These varieties combine high yields with good storability and a consumer quality that is equal or better than the standard varieties. Leaving out all fungicidal treatments on these varieties in summer leads to infections by summer diseases like Soothy Blotch (SB), and in a far lesser extend by Flyspeck. Depending on production region, variety, year and local situation SB-infection of fruits can be up to 100% incidence and severity. Due to increasing demands on the fruit skin quality of organically produced fruits infected fruits can not be sold. Of the substances allowed crop protection within the European regulation on organic production EU 2092/91 Annex 2, Lime sulfur and potassium soap seem to be the most effective materials for the control of SB. However the effectiveness of the control with these materials varies widely between trials (Fuchs 2001, 2002, Kiensle 1995, Zuber, M. 1997) In 2003 we developed a simulation model for SB in order to find the most important moments for fungicide application. (Trapman 2003) The model was build on published data from research in USA as there are hardly any data published on biology and epidemiology of SB in Europe. In 2003 and 2004 spray schedules based on model output where quite effective, however in 2005 model output did not agree to the field observations, and the output seemed to be very sensible for the logger data used as input. (different types of stations, placement and LW-sensor type). For validation purposes from 2002 to 2005 field observations where made on the occurrence of SB in organic orchards. The results of these observations are present in respect to: Bio Fruit Advies Dorpsstraat 32 4111 KT Zoelmond Netherlands

1. The moment that first SB symptoms where found 2. The development of the SB epidemic within a year 3. The disease development in the same orchard over years

Material and Methods Observations where made in organic and in untreated orchards in The Netherlands and Belgium, and in one organic orchard just across the boarder in Germany. The number of orchards included in the survey increased during the years. In the organic orchards observations where made in blocks of at least 4 tot 6 rows wide, and at least 20 trees long that where left untreated during summer. In most cases the same untreated plots where used several years. Al observations where made on the Vf scab resistant apple variety Topaz. The orchards where visited regularly during summer. For each sample 200 to 500 randomly chosen fruits where examined for Sooty Blotch symptoms. Disease was scored using the scale proposed by Hartman (Hartman 2000): 0 = no symptoms 1 = trace 90% of the fruits. Over the years, with a few exceptions, disease level in the same orchard roughly stays at the same range. The figures in table 2 are only valid for the monitored plots in the orchard, as within the same orchard disease incidence may vary widely. In the same orchard trees in the shade of wind breaks develop more disease than is average for the orchard. Overall in the dataset there is however no correlation to the distance to hedgerows or other possible sources of outside inoculum. These observations lead to the conclusion that he occurrence of SB on Topaz is strongly depending on local conditions. These might include as well local disease pressure and microclimatic conditions. Local disease history seems a strong indication for infection risk.

Table 2: Percentage fruits with Sooty Blotch symptoms at harvest in untreated orchards or untreated orchard plots. Variety Topaz.

Orchard

2001

2002

2003

2004

2005

Zoelmond 67 100 64 92 Lobith 10 2 5 Randwijk 35 13 3 0**) Rees 16 22 Mijel 3 27 Nispen 100 Varik 30-50 Kraggenburg