THE INFLUENCE OF THE HOST ON THE DIMENSIONS OF THE PLANT PARASITIC NEMATODE, DITYLENCHUS DESTRUCTOR

Measurement data and body ratios from eleven populations of Ditylenchus destructor are presented. The data for lengths of males and females have been analysed for variance, and many differences between population means are shown to be significant. The essential importance of structural characters and the care with which sizes and dimensional ratios must be used, in the identification of a species, is emphasized.     

THE SUSCEPTIBILITY OF POTATO VARIETIES TO INFESTATION BY THE EELWORMS DITYLENCHUS DESTRUCTOR AND D. DIPSACI

Twenty-five varieties of potato in common commercial cultivation were found to be susceptible to tuber attack by potato-derived populations of Ditylenchus destructor under field and pot experimental conditions. Stunting and leaf deformation may also be caused by the eelworms but appear less consistently. A race of D. destructor from mushroom spawn had almost no effect on potatoes. Various races of D. dipsaci can reproduce in the shoot tissue of potato, sometimes causing damage. One population of this stem eelworm produced lesions on the tubers.     

INVESTIGATIONS ON RESISTANCE OF OATS TO STEM EELWORM, DITYLENCHUS DIPSACI KÜHN

A procedure adopted for testing many varieties of oats and segregating progenies for reaction to attack by stem eelworm ( Ditylenchus dipsaci Kühn) is described, and its merits and limitations discussed in relation to the breeding of resistant varieties.
A study of some 250 forms of oats revealed new sources of resistance in cultivated and wild species. New sources of resistance in the cultivated species Avena sativa were found only in winter types, and in the other hexaploid species in forms belonging to A. byzantina (C) Koch, and the winter wild oat A. ludoviciana Dur.
Segregate progenies in advanced generations could be selected in the field by their reaction in a single drill in the first year followed by a head-row progeny test in the next season. Reaction was sufficiently well defined to identify the truebreeding resistant and susceptible lines.
The inheritance of reaction to stem eelworm in crosses involving Grey Winter and susceptible types depended on a single factor pair with resistance dominant.     

DRY-ROT DISEASE OF THE POTATO

Inoculation of wounded tubers at intervals throughout the storage season showed that, after harvesting, resistance to infection by F. avenaceum was maintained for a longer period than resistance to F. caeruleum , although eventually the tubers became equally susceptible to both fungi. Tubers were less readily infected through clean-cut than through scarified wounds. Resistance to infection was greater when an interval elapsed between wounding and inoculation than when the wound was inoculated immediately; this was related to an increase in the intrinsic ability of the cells near the wound to resist infection rather than to suberization of the wounded surface or wound periderm formation.
Histological studies showed that F. caeruleum grew through the intercellular spaces and that the adjacent host cells remained alive, often for considerable periods, whereas F. avenaceum killed and penetrated the cells with which it came into contact. Restriction of incipient or established lesions caused by F. caeruleum and of incipent lesions caused by F. avenaceum was associated with suberin deposition on the host cell walls and in the intercellular spaces; in established lesions caused by F. avenaceum , restriction was attributed to an increase in the intrinsic resistance of the adjacent host cells, similar to that found near a wounded surface.     

THE DEVELOPMENT OF POTATO SKIN-SPOT DISEASE

A study of the development of skin spot in the potato variety Kerr's Pink has shown that humid conditions, both before and after infection by Oospora pustulans , increase the incidence of the disease. The results of inoculation experiments have indicated that infection of the tuber does not take place much before the normal time of lifting.
It has been found that the pathogen may enter the tuber by way of the lenticels, and that invasion of the cortex and the production of a cork cambium takes place shortly after infection. Some damage to buds was observed within 16 days of inoculation. Although the affected pocket of cortical tissue soon dies, discoloration of the periderm is not apparent until much later. It is suggested that this may be the reason for the comparatively long period which elapses between infection and the appearance of skin-spot symptoms.     

陈化马铃薯切片内源乙烯和抗氰呼吸途径发生,运行的关系

马铃薯(Solanum tuberosum L.)切片在24h陈化期间,总呼吸速率(V_t)明显升高,交替途径容量(V_(alt))增加更为显著。交替途径实际活性(ρV_(alt))及其对V_t的贡献(ρV_(alt)/V_t)在陈化初期上升,12h后基本保持稳定;相应交替途径运行系数ρ值随陈化进程不断下降。陈化切片内源乙烯产生的时间曲线与V_(alt)的变化趋势一致,而与ρV_(alt)的趋势不同。用乙烯相关效应剂进行实验,发现促进乙烯产生的ACC及Cu~(2 )可促进V_(alt)的发生,而抑制乙烯产生或其生理作用的Co~(2 )及Ag~ 则可部分抑制V_(alt)的发生。所有上述效应剂虽然对ρV_(alt)及ρV_(alt)/V_t值在12h前有所影响,但都不能改变切片陈化期间交替途径ρ值不断下降的趋势。这些结果表明,内源乙烯对马铃薯切片陈化过程中交替途径的实际运行稍有影响,但对抗氰容量的诱导发生具有重要的调控作用。