Evaluation of pollen storage strategies in Cucurbita pepo L.

Male flower shortage is a major reason of insufficient pollination in zucchini (Cucurbita pepo L.), leading to pointy fruits that rot quickly in the post-harvest phase. Harvesting pollen grains early in the season, when male flowers are abundant, followed by storage and subsequent manual pollination, could be a solution, provided that high pollen viability is maintained. Therefore, insight in parameters affecting pollen viability during anthesis and under storage are needed to advice growers on the most successful pollination strategy. Pollen viability was determined using Fluorescein Diacetate (FDA) and digital image analysis performed with Image J. Under practice conditions, viability started to diminish three hours after anthesis. At flower closing, only 60% of pollen grains remained viable and the next morning no more than 10%. In order to assess the impact of storage on pollen viability, pollen grains were subjected to different relative humidities and storage times. Stored at 20°C and 80% RH or lower for four hours, at least 90% of the pollen turned out to be non-viable. At 20°C and 90%, RH pollen viability remained above 50% for eight hours. When female flowers were pollinated with stored pollen (five days or longer), none of the fruits set sufficiently to be marketable, while fresh harvested pollen resulted in 90% marketable fruits. Therefore, pollen should be harvested and applied on the stigma within three hours after anthesis in order to ensure good fruit setting. Pollen could not be stored longer than eight hours without severe loss of viability or pollinating capacity

Epidemiology of pseudomonas cichorii, the cause of lettuce midrib rot

Bacterial midrib rot, caused by Pseudomonas cichorii, has become a serious threat to the production of greenhouse butterhead lettuce (Lactuca sativa L. var. capitata) in Belgium. Currently, there are no strategies for controlling this pathogen. Therefore, greenhouse experiments were conducted to obtain more knowledge about the epidemiology of P. cichorii on butterhead lettuce. Greenhouse butterhead lettuce becomes susceptible to lettuce midrib rot infections at head formation, and a single overhead irrigation with water containing 102 CFU/ml P. cichorii was sufficient to cause disease. The use of surface drip irrigation instead of overhead sprinkler irrigation significantly reduced midrib rot incidence in the greenhouse. P. cichorii isolates can be divided into subgroups based on BOX-PCR genomic fingerprinting, with isolates belonging to subgroup C1 and C2 being more virulent than those of (or related to) subgroup C3. P. cichorii infections with distinct symptoms comparable to midrib rot have also been observed on field-grown crisphead lettuce in California and Japan which, respectively, are referred to as 'varnish spot' or 'tar'. We showed that symptom expression is strongly influenced by the lettuce cultivar group, irrespective of the P. cichorii isolate, resulting in varnish spot/tar on crisphead lettuce and midrib rot on butterhead or cutting group lettuce

Quantifying plant responses of a tomato crop under a thermal screen

Although thermal screens were already introduced more than twenty years ago in Western European glasshouse horticulture, growers were discouraged to install them in their glasshouse, since they were not able to quantify the crop's behaviour under a thermal screen. Nowadays, crop monitoring techniques have become commercially available, which might aid growers to support their decision to adapt their climate control set points once a thermal screen is installed. In this study, leaf temperature, stem diameter and sap flow measurements were performed on glasshouse tomato plants (Solanum lycopersicum L. 'Clothilde') grown inside two compartments in Beitem, Belgium during spring 2008. In one compartment, a movable and semi-transparent thermal screen was used during the night. Differences in leaf temperature between the two compartments were observed, which were mainly caused by differences in air temperature, although differences in radiative energy losses also had to be taken into account in the upper canopy layer in the case of cold nights. Based on these results, it is advised to include leaf temperature measurements in research concerning the effect of the use of thermal screens. Additionally, a reduced water uptake was observed in the morning when the screen was still closed, whereas during the evening hours no difference in water uptake could be revealed. However, when the thermal screen was closed in the evening, stem diameter measurements indicated that the internal water storage pools were restored more quickly under the screen. It is, however, acknowledged that these effects were only observed on sunny days. Consequently, in addition to leaf temperature measurements, sap flow and stem diameter measurements can provide growers and researchers useful feedback from the crop itself, when thermal screen application strategies are to be evaluated. As a general conclusion, it was stated that these crop monitoring techniques are imperative when novel techniques are to be introduced in glasshouse horticulture in a sustainable way

Glasshouse-specific occurrence of basal rot pathogens and the seasonal shift of Rhizoctonia solani anastomosis groups in lettuce

Basal rot is a common disease in Belgian lettuce, which is mainly controlled by fungicides and chemical soil disinfestation. A seasonal appearance of the basal rot pathogens: Rhizoctonia solani, Sclerotinia spp., Botrytis cinerea and Pythium spp. has been reported, but lettuce growers use standard spraying schemes, irrespective of the occurrence of the pathogen. Due to stricter regulations and environmental concerns the superfluous use of fungicides should be omitted. We investigated if the use of fungicides could be reduced by only controlling the active pathogens. Therefore, lettuce was continuously grown in three glasshouses without any fungal disease control and the active pathogens causing basal rot were identified. The occurrence of basal rot pathogens appeared to be glasshouse specific and the different basal rot pathogens were active throughout the year. However, a seasonal appearance of R. solani anastomosis groups and Pythium spp. was observed with AG4-HGI and Pythium ultimum active at higher temperatures and AG2-1, AG-BI, AG1-IB and Pythium sylvaticum at lower temperatures. We report for the first time the isolation of AG-BI from infected plants. Each R. solani anastomosis group had its own optimal growth rate in vitro. Differences in pathogenicity between R. solani anastomosis groups were observed on detached leaves. AG1-IB and AG4-HGI were most pathogenic, followed by AG2-1 and AG-BI. These results show that the fungicide spraying scheme should be adapted to the occurring pathogens in the glasshouse. This information is of high importance in developing a sustainable control strategy for basal rot pathogens