No abstract

The influence of the nitrogen‐content of apple leaves on the development of the fruit tree red spider mite (Metatetranychus ulmi (Koch)) was studied under laboratory and semi‐field conditions. The experiments in the laboratory showed that a higher egg production per female in 24 hours, a lower mortality rate of the eggs, larvae and nymphs, and a shorter period of development of the male as well as the female mites was obtained on the leaves with a high nitrogen‐content. However, these differences were statistically significant only in the case of egg production and mortality rate. From the experiments under semi‐field conditions it became evident that the egg‐female ratio on leaves with high nitrogen content originating from manured rootstocks was also significantly higher than this ratio on leaves with a low nitrogen content. In both series of experiments the differences gave a factor of 1.6. The experiments confirm the results, we obtained earlier from perennial field trials where an effect of manuring of the apple trees on the development of M. ulmi was found. It is concluded that the mortality of predators of M. ulmi, caused by chemical treatment, is not the only reason why the red spider mite populations are stimulated in well kept orchards. The higher level of nitrogen content of the leaves, brought about by cultural practice, in particular manuring, may be of considerable importance also. ZUSAMMENFASSUNG Der Einfluß der Düngung in Obstgarten auf die Populationsdichte von Metatetranychus ulmi (Koch). Der Einfluß des Stickstoffgehaltes der Blätter auf die Entwicklung der Roten Obstbaum‐Spinnmilbe, Metatetranychus ulmi (Koch), wurde unter Laboratoriums‐ und halbfeldmäßigen Bedingungen geprüft. Steigender Stickstoffgehalt ergibt eine höhere Eiprodukten pro Weibchen und je 24 Stunden, eine geringere Absterberate der Eier, Larven und Nymphen und eine höhere Entwicklungsgeschwindigkeit sowohl der Männchen wie der Weibchen. Im Falle der Eiproduktion und der Mortalitätsraten waren die Unterschiede gesichert. Unter den Halb‐Freilaudbedingungen wurde es deutlich, daß das Ei‐Weibchen‐Verhältnis bei hohem Stickstoffgehalt der Blätter gedüngter Wurzelstöcke ebenfalls signifikant höher war als auf Blättern mit niedrigem N‐Gehalt. Sowohl im Labor wie im Freien betrug der Faktor 1,6. Diese Feststellungen bestätigen die Ergebnisse langfristiger Feldversuche Post's über die Wirkung der Düngung von Apfelbäumen auf die Entwicklung von Metatetranychus ulmi. Aus diesen Befunden ergibt sich ganz deutlich, daß die Abtötung der Feinde von M, ulmi nicht der einzige kausale Faktor für die Zunahme dieser Milbe ist. Der Anstieg des Stickstoffgehaltes der Blätter, der durch die Kulturmaßnahmen, besonders die Düngung, hervorgerufen wird, kann ebenfalls von beträchtlicher Beteutung sein.

Breeding for a certain trait is only possible when the phenotypic variation that is caused by genotypic variation can be estimated. For traits that strongly depend on environmental conditions, this can be extremely difficult and knowledge and collaboration with experts from other disciplines becomes essential. A wellknown example is breeding for disease resistance. Here, we describe a similar approach to assist breeding against adverse leaf deformations that severely reduce the ornamental value of some chrysanthemum (Dendranthema × grandiflora) genotypes during greenhouse cultivation in winter. These leaf deformations occur rather unpredictably, but seem to be related to the increased use of assimilation light. To breed against this trait knowledge is needed (i) about inductive environments in which sensitive and insensitive genotypes are distinguishable, and (ii) about the physiological background associated with leaf epinasty. In this paper hypothetical physiological factors and mechanisms are discussed, which may mediate effects of light spectrum and greenhouse climate on leaf epinasty. One factor involved could be starch accumulation, since leaf epinasty usually aggravates after disbudding -a practice that most probably alters the sink-source balance. Additionally, light spectra can affect the circadian clock and thereby disturb starch synthesis and breakdown resulting in accumulation. Both within and independent of this process, plant hormones such as auxin and ethylene may play a role in leaf epinasty. This theoretical framework will be used to further investigate the physiological background of leaf epinasty and to come up with a test in which susceptibility for leaf epinasty can be assessed.

THE EXPERIMENTAL ORCHARDApple trees, Lombartscalville on r o o t s t o c k M I, were p l a n t e d in 1953 at a distance of 5 × 5 m on a shallow m a r i n e silty clay loam soil. After two years, trees of the s a m e v a r i e t y and r o o t s t o c k were i n t e r p l a n t e d so t h a t the distance * Former biologist at the Experiment Station for Fruit Growing, Wilhelminadorp (Z) ; at present Lady Principal of the Secondary Horticultural School for Girls, Rijswijk (Z.H.).

No abstract