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Brown Spot (Stemphylium versicarium)

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ImageBrown spot on pear are caused by the pathogenic fungus Stemphylium vesicarium, which also causes disease on garlic, leek, onion and asparagus. On pear, the fungus infects leaves, fruits and to a lesser extent twigs. The resulting necrosis and fruit rot is caused by fungal penetration of stomata and lenticels and the production of chemical compounds which forces the host to kill the cells in the infected area,  causing the brown spots. Brown spots causes severe damage especially in southern Europe. However, the disease was also found in the Additionally, the predominant pear cultivar in the Netherlands and Belgium, is very susceptible (Montesinos et al., 1995a).

The influence of temperature and wetness duration on conidial infection by S. vesicarium on pear has been studied previously (Montesinos et al., 1995b). The findings there led to the development of a brown spot forecasting system (Llorente et al., 2000). Climatical conditions in Europe are differing form South to North. In Southern Europe we can expect thunderstorms even during warm periods, whereas in Northern Europe cool raining periods lasting for several hours can occur even during mid summer. It is obvious that the forecasting system should be revalidated or even adapted for use under different climatical conditions.
The moist periods leading to Stemphylium versicarium infections on pear have to be very long following this model. The model goes back to the work carried out by Llorente, I., Vilardell, P., Moragrega, C. and Montesinos, E. and the adoption to electronic weather stations done by A. Boshuizen, P.F. de Jong and B. Heijne from Netherlands. This moist periods can be disrupted. The length of disruption depends on the relative humidity or the vapor pressure deficit.

 

In the calculation of the model of FieldClimatethe disruption can last for ever as long as the relative humidity is higher or equal to 75%. With a relative humidity in between 65% and 74% the disruption of the moist period can last for 12 hours. If the relative humidity is in between 55% and 64% the disruption can last for 9 hours. If relative humidity is in between 45% and 54% the disruption can last for 8 hours. If relative humidity is in between 35% and 44% the disruption can last for 6 hours. If relative humidity is below 34% the disruption can last for 4 hours.
The curves showing the progress of light, moderate and severe infections can be used as action thresholds for orchards with a differing disease history. In orchards with more than 1% disease incidence on fruit chemical control of the disease should be done on all light infections. In orchards with less than 1% of disease incidence on fruits chemical control should be done on all completed moderate infections. And in orchards which had up to now no occurrence of Stemphylium control methods should be started if severe infections will reach 100%.

Conditions:

Temperature: 8°C - 38°C

Leaf wetness > 0 or rel. humidity >90%

Factor: 600; max: 60000 (100% infection).

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(c) Dr. Heinrich Denzer, Pessl Instruments GmbH, Weiz, 2009