To achieve target yields, crop regulation measures in 'Conference' pears may vary from year to year and may embrace measures to stimulate flower bud development and enhance fruit set, but often also chemical thinning of excess fruit. In 'Conference' the value of the crop at harvest strongly depends on the size distribution of the fruit. The aim is to harvest as many pears as possible with fruit diameters of 65 mm or more, since these sizes provide better economic returns than do smaller fruit. In years with ample flowering and good conditions for pollination, fruit set frequently becomes excessive and thinning is needed to realize the target crop load and the desired fruit size. Hand thinning is labor intensive and thus expensive. Therefore, chemical thinning is being examined as a cheaper alternative. Ethephon applied at 12 to 14 mm fruit diameter also thinned 'Conference' pears, but was somewhat less effective than the combination of BA and NAA. The percentages of fruit having a diameter >65 mm were significantly increased up to 80 to 90% of the yield for those treatments that thinned the trees to the target fruit load of about 110 fruit per tree.

The increasing difference in the market value of small and larger sized 'Conference' pears (>65 mm) and the high labour costs for hand thinning, makes it interesting for growers to find a cheaper and reliable method for thinning pear trees. In 2007, 2008 and 2009 trials were carried out to test the thinning efficacy of several chemicals. Fruit set was only significantly reduced by a two-fold application of ammonium thiosulphate (ATS) or by a tankmix application of 6-benzyladenine (BA) and 1-naphtalene acetic acid (NAA). Unfortunately, russeting of 'Conference' pears was augmented by the treatments with ATS during bloom. Tankmix applications of NAA + BA, applied at an average fruit diameter of 8.8 mm, resulted in a severe overthinning of the trees in 2008. However, the same treatments applied at an average fruit diameter of 14.7 mm had much less of a thinning effect. In 2009 adequate thinning was obtained with lower concentrations of BA+NAA applied at 8-10 mm fruit diameter. Surprisingly, the reduction in fruit set by BA+NAA did not result in a proptional increase in the average fruit weight at harvest. In some treatments average fruit weight was even reduced compared to that of the handthinned trees, a size-reducing effect that was attributed to NAA. In conclusion, BA+NAA seems to thin 'Conference' pears more strongly than BA alone. However, the addition of NAA to BA has the disadvantage of a diminishing effect on the increment of fruit size, i.e. fruit size increased less than expected on basis of the reduction in crop load. Additional research will be carried out to further optimise the developmental stage, weather conditions, and concentrations at which a BA and NAA must be applied to obtain the desired level of thinning and gain in yield percentage of fruits over 65 mm in diameter.

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European fruit tree canker, caused by Neonectria ditissima, is an important disease of pome fruit worldwide. Apple cultivars differ in their levels of susceptibility to N. ditissima. In order to design an effective plant resistance test, we examined the effectiveness of two resistance parameters: infection frequency and lesion growth. Both parameters were evaluated in parallel tests using 10 apple cultivars in three experimental years, applying seminatural infection of leaf scars (infection frequency) or inoculation of artificial wounds (lesion growth). We compared six parameters for lesion growth, of which a new parameter, lesion growth rate (LGR), appeared to be the best with respect to reproducibility and statistical significance. LGR is defined as the slope of the regression of lesion size versus time. The slope was estimated for each lesion, employing a common start date and a lesion-specific end date determined by the girdling of the lesion. The two parameters (infection frequency and LGR) were examined in separate experiments and in three successive years, and provided complementary information and resulted in reproducible conclusions on the relative resistance levels to N. ditissima of the tested cultivars. The presented methods can be used to develop strategies for the control of European fruit tree canker (e.g., in the breeding of new apple cultivars with high levels of resistance to N. ditissima).

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