Didymella bryoniae on glasshouse cucumbers

English. The increasing incidence of fruit rot, caused by Didymella bryoniae , in glasshouse cucumbers in the Netherlands gave rise to a study of the biology of the pathogen, the epidemiology of the disease, and control methods. Stems, leaves and growing tips may be attacked, and fruits may be infected internally as well as externally. Internal infection of fruits starts at the blossom end and is initially hard to observe externally. With external fruit rot, lesions occur on any place of the fruit, predominantly in the post harvest period.The survival of the fungus outdoors and in glasshouses was studied. Outdoors, the fungus was able to survive winters with 14 days of frost. A temperature above 5 °C was needed for growth and fructification of the fungus. Diseased plant residues remained an infection source as long as they were not decomposed. Deliberate introduction of diseased plant debris in the glasshouse caused an earlier appearance of symptoms, and increased the severity of the disease. Hygienic measures are advised to eliminate infection sources.The number of daily trapped ascospores showed a peak after sunset, both outdoors near diseased plant debris and in glasshouses with diseased plants. Watering the plants in the glasshouse favoured ascospore release. Under controlled environmental conditions, ascospores were not released at a high relative humidity or after a change in relative humidity. They could only be trapped after wetting of the substrate. The first ascospores in glasshouses were trapped at about the same time the first symptoms on the plants appeared.Wounding was essential to achieve external fruit rot. The degree of rot was increased by more severe wounding. The rotting of the fruits increased progressively from 12 to 23 °C; it decreased at higher temperatures and no rot at all occurred at 35 °C. To reduce rotting without diminishing the shelf life, fruits have to be stored at 12 °C in the post harvest period. Relative humidity during storage had no effect on fruit decay. A low nitrogen fertilization of the crop decreased the degree of fruit rot but reduced also the production. The difference in fruit rot in the light and in the dark had no practical significance.Internal fruit rot was achieved by inoculation of the open flower; however, the majority of the fruits escaped infection. It took more than two days for the fungus to reach the fruit after infection of the style. Neither the method of inoculation, nor amending nutrients to the inoculum suspension influenced the incidence of internal fruit rot. Slight wounding by removing parts of the flower had no effect on the incidence of internal fruit rot, whereas severe wounding by blossom excision reduced it with 75%. Inoculation of wilted flowers resulted in 60% less infection than inoculation of fresh flowers. Some powdery mildew resistant cultivars showed resistance to internal fruit infection. This resistance was associated with a long style and a short flowering period. However, no obvious effect of the flowering period within a cultivar on the incidence of internal fruit rot was observed. Growing plants under drought stress markedly increased internal fruit infection. Duration of fruit growth and fruit thinning had no effect on fruit infection. Relative humidity had no effect either. For flower infection no free water seems to be needed. It is not yet clear whether some kind of defence mechanism to fruit infection existst from which the fungus may escape under certain circumstances.Under controlled environmental conditions infection of growing tips and leaves was rare at 60% r.h., it increased at 95% r.h. and was most serious if the plants were kept wet. A period of 1 hour of free water was sufficient for the initial stage of infection. For further expansion of the disease, leaf wetness was required. A high relative humidity did not predispose leaves to infection. Wounding was essential for infection of older leaves, but not for infection of young plant tissue. A higher conidial concentration increased infection. The effect of leaf wetting on the number of infected sites was comparable with the effect of a tenfold conidial concentration.In glasshouses with cucumbers grown in a commercial way, a lower night temperature in the pre-inoculation period, between 12 and 24 °C, increased the disease incidence later on in the season, whereas no postponed effect of the day temperature, between 17 and 26 °C, was observed. The daily maximum relative humidity was about 10% lower in glasshouse compartments with a minimum ventilator opening than in compartments without a minimum ventilator opening. This decreased the disease incidence on uninoculated plants to 35% and on fruits to 40%. The spread of the disease from inoculated plants to uninoculated ones was more rapid under humid than under drier conditions. However, production was higher under more humid conditions.Disease incidence was higher at an interval of 5 °C between heating and ventilation than at an interval of 2 °C. Little ventilation with an electric fan at an interval of 5 °C between heating and ventilation hardly influenced the incidence of the disease.The influence of transition to day temperature 3 h before, 1 h before, 1 h after and 3 h after sunrise on the incidence of the disease was studied. The later the transition to day temperature took place, the longer the periods with high relative humidity were maintained, the longer the periods the fruits stayed wet by condensation of water and the higher the incidence of the disease on inoculated plants and fruits was.Inoculation of plants resulted in a high infection pressure and in an increase of the disease incidence on plants and fruits as well as in an increase of the number of misshapen fruits. Inoculation reduced the number of internodes by 18% after four weeks and the number of fruits by 10%.The male and female flowering cultivar Spotvrije was as susceptible to stem and fruit rot as the female flowering hybrid Farbio. The tested breeding material showed plant resistance, both in the young and mature stage, and resistance to external fruit rot if the fruits were slightly wounded. Resistance to internal fruit rot was not observed.Not always a correlation was found between the activities of fungicides in vitro and in vivo. Testing fungicides on young inoculated plants under humid conditions was a reliable method. Best control was obtained with benzimidazole fungicides, tolylfluanid and chlorothalonil. Ergosterol biosynthesis inhibitors, used to control powdery mildew, and dicarboximides, used to control Botrytis , gave also some control of D. bryoniae . Frequent, preferably weekly sprays were necessary to obtain a reasonable effect.The disease has to he controlled by a combination of hygienic measures, chemical control and manipulation of the glasshouse environment, especially by preventing humid conditions. A contribution of resistance breeding is expected in the future.

Dutch; Flemish. Het toenemend optreden van vruchtrot, veroorzaakt door Didymella bryoniae, in de intensieve glasteelt van komkommer in Nederland was aanleiding tot nader onderzoek over de biologie, de epidemiologie en de bestrijdingsmogelijkheden van de schimmel. Hierbij werden de invloed van de relatieve vochtigheid tijdens de groei en bewaring, dikte van de vruchtschil, verwonding, watergebrek, groeisnelheid, vruchtdunning, dag- en nachttemperatuur en de werking van fungiciden op vruchtrot nagegaan