Cucumber Powdery Mildew: Numerous vegetable crops are susceptible to powdery mildew, but cucurbits are one group that are severely affected. It is probably the most common, conspicuous, widespread and easily recognizable disease of cucurbits. Like other powdery mildew diseases, its symptoms are characterized by the talcum-like, powdery fungal growth that develops on both leaf surfaces, petioles and stems but rarely on fruits. Podosphaera fusca (also known as Podosphaera xanthii or Sphaerotheca fuliginea) and Golovinomyces (Erysiphe) cichoracearum are identified as the main agents of cucurbit powdery mildew. The disease provides one of the most important limiting factors for cucurbit production worldwide. In the absence of chemical, biological control or the use of tolerant/resistant varieties the disease can cause yield reduction (as much as 40%) and impair fruit quality. Moderate temperatures, reduced light intensity i.e. partial shade and succulent plant tissue promote disease development. The disease is spread via spores (conidia) to other plants on air currents. Although favouring dry conditions, spore release (disease dissemination) can occur across high humidities and infection can occur without the necessity of a water film on the plant surface. In Europe, G. cichoracearum has been reported during early season croppings preferring a dry climate whilst P. fusca dominates during the summer months as humidity is increased. It is generally regarded that P. fusca is the causative agent in the UK however this project would look to determine G. cichoracearum occurance.
The pathogen is unable to survive particularly long in the absence of a living host. However once spores are released into the air they can remain viable for a few days. The length of time between infection of the host plant by the spore and symptom appearance can be as short as 7 days but can take longer than this if conditions are below optimum for the infection process. At present, growers only know that powdery mildew is present once symptom development is observed and the disease is established within the crop. The application of fungicides is the principle practice in cucumber cropping for mildew control. However powdery mildew pathogens have a high potential for fungicide resistance and there is a need for control programmes to be less reliant on blanket spray applications.
Gummy stem blight (Black stem rot) of cucumber is caused by Mycosphaerella melonis syn. Phoma cucurbitacearum (syn. Didymella bryoniae) and is of worldwide importance, causing significant economic damage of glasshouse cucumber & other cucurbits, including outdoor crops. It causes extensive stem & leaf infections which when severe can debilitate or even kill plants. Like the powdery mildew pathogen, airborne spores are produced and involved in spread of the disease. The infection of flowers & developing fruit leads to fruit rot. Often disease symptoms are not visible until the fruit is marketed. This leads to rejection and reduced retailer & consumer confidence in the product. Fungicides are used routinely in an attempt to suppress the disease and prevent plant and fruit losses. The fungicides that are available in the UK for use in cucumber production (primarily for powdery mildew control) provide only a partial suppression or reduction of the disease.
Under a AHDB Horticulture funded project (PE001) a range of alternative fungicides have been assessed for their efficacy in control of the disease. Monoclonal antisera and a laboratory based ELISA test has been developed to monitor glasshouse aerosols for M. melonis spore disease transmission events. The timed application of control measures was made during periods of peak spore production (>2000 spores m3) to provide improved fungicide efficacy. However as a result of postal delays the time period between spore monitoring, data capture, interpretation, communication and fungicide spray application was too long . To provide this information in a more timely way the laboratory based immunoassay test (ELISA) could be transferred to a lateral flow ‘on-site’ format for direct grower or consultant use.
Lateral flow immunoassays are used for qualitative and semi-quantitative detection of target analytes. Lateral flows consist of a carrier material containing dry reagents that are activated by applying a liquid sample. Movement of this liquid allows passage across various zones where molecules have been attached that exert specific interactions with target analytes. Results are generated with 5 – 10 minutes with the formation of a control and test line as appropriate to the sample and the test type. They are designed for single use, can provide a multiplex test platform and, are available commercially for a wide range of applications. The most well known test of this type is the Unilever Clear Blue Pregnancy Test Kit. More recently they have become increasingly important in the diagnosis of plant pathogens. A range of lateral flows have been developed through AHDB Horticulture funding to measure bio-aerosols for a range of plant pathogens. These include Powdery mildew (Erysiphe cruciferarum) and ringspot (Mycosphaerella brassicicola) in field brassica cropping systems.
Information on plant pathogen spore concentrations (inoculum load) in bio-aerosols can be utilised within an integrated disease management system. In Holland, an environmental model is under evaluation for control of Mycosphaerella in cucumber (A. Dijk, pers. comm.). Future work should look to integrate the environmental disease forecast with disease concentration. This would provide information on when airborne pathogens present at a concentration required for infection of the crop and whether the environmental conditions were conducive for infection to occur. In this way an informed decision could be made to apply the appropriate control measure in an effective and targeted way – and in advance of infection occurring in the crop. This approach may however not necessarily be appropriate for powdery mildew throughout the entire cropping period, especially where the environmental conditions during the growing season tend not to be limiting but it may be useful in helping delay the initial onset of infection thus reducing the number of sprays required and minimising the risk of resistance developing in the pathogen population. The development of diagnostic probes to selectively measure bio aerosols could nevertheless provide a rapid ‘grower’ on-site test with capability to discriminate and diagnose spore concentrations of both Mycosphaerella and powdery mildew in glasshouse propagation and growing crops. This would form part of an integrated disease management strategy to control disease from the outset and ahead of visible symptom expression.
A short follow-on project was commissioned in order for additional development work to be carried out to ensure the grower test was stable. Further work to characterise 'myco' isolates for genetic variability and their sensitivity to a range of fungicides was included in the project.
A fully-stable lateral flow device has now been successfully developed. Growers who have spore trapping technology in their glasshouses can now use the test to measure the risk of infection in their crop from myco spores, getting a result in just 10 minutes. We also found that isolates collected from 2 geographically separate areas were genetically identical, suggesting that no difference in aggressiveness or virulence is likely. The collected isolates were found to be sensitive to all of the fungicides tested, although there were some level of variability seen. However, this was not deemed likely to suggest the development of resistance to approved treatment options.