Asparagus: Optimising Urea application rates and timings to reduce purple spot disease on emerging new crop and ferns

The Problem:

Stemphylium purple spot of asparagus, caused by the fungus Stemphylium vesicarium, occurs on spears during the harvest season, and reduces productivity and spear quality. The disease develops predominately on the asparagus ferns after harvest, affecting main stems, secondary branches and needles where survival structures (pseudothecia) of the fungus are produced. These overwinter on fern debris and acts as an inoculum source to infect the following crop.

Once purple spot lesions are present, asexual spores (conidia) can be produced and are readily spread by wind and watersplash. This can lead to a rapid increase in disease which can be very difficult to control in the fern canopy once established. Fern debris from the previous season is often present on the soil surface as spears emerge. Burial of the debris by ridging up after the crop has senesced can reduce the risk of Stemphylium spore release the following spring. In older crops, where crowns and roots develop closer to the soil surface, this can be difficult due to the risk of damage to the extensive root system and debris may remain. Heavy rainfall can also cause the debris to become exposed again.

Field trials in project FV 341b (Asparagus purple spot: field evaluation of Urea application rates and timing, and Perlka {calcium cyanamide} for control of Stemphylium vesicarium) demonstrated that purple spot can be reduced on the ferns by a pre-harvest application of urea. It also decreased S. vesicarium spores being released from asparagus fern debris. An additional application of urea could be useful post-harvest after the debris has been disturbed by machinery and footfall to give a clean up before the ferns develop. The benefit of this extra application will be evaluated in the 2014 trial. There are also liquid Urea products available (Nufol20 and Nuram37) which may be more convenient to use than dissolving granular Urea on a large scale. The aim of the field trial part of this project is to determine the most suitable rate, timing and form of urea for commercial application in field crops by examining their effect on (i) level and persistence of spore production from debris and (ii) occurrence of purple spot on spears and ferns.

Water volume, adjuvants and other similar products can improve the spread and dispersal of products such as urea, and in turn improve efficacy. The aim of the pot/tray trials is to determine the effect of a wider range of water volumes and products than can be included in the field trial on reducing the level and persistence of spore production from debris. As shown in FV 341b, decreases in spore counts can be related to decreases in purple spot in the crop, and can further refine the treatments in the field.

Benefits to industry:

• Information to reduce the risk of Stemphylium infection on asparagus spears and ferns;
• Potential for improved non-pesticide control of Stemphylium consistent with the Sustainable Use Directive (SUD) objectives;
• Reduced need for intensive fungicide program on ferns if the epidemic is delayed through inoculum reduction.

Aims and objectives:

Project aim(s):