A two year project that investigated the influences of weathering, insect honeydew and fungal agents on cotton colour grades, yarn and fabric quality parameters provided a better understanding of these factors and the conditions under which they occur. Extended weathering in the field, particularly with successive rainfall events totalling 100 mm, was detrimental to cotton colour, reducing colour from base grade to 41 and beyond. In both years of the project this equated to 10 weeks in the field from the time of first open boll. Moist, overcast conditions for 4 -7 days were usually followed by a drop in colour grade. Short, sporadic rainfall events followed by sunshine did not cause colour degradation. The humidity they provided could, however, further the development of sooty mould fungi which grew on honeydew contaminated cotton. While sooty mould fungi can grow on the natural sugars of immature fibres in the absence of moisture, both the presence of insect honeydew and rainfall strongly promoted their development, reducing cotton colour from base grade to 41 to 71. The magnitude of the colour drop depended on the amount of black spores on the boll surface. Increasing development of sooty mould fungi was also correlated with a reduction in sugar concentration on bolls, providing some evidence of the ability of fungi to remove sugar from honeydew contaminated cotton. Sooty mould spores in cotton fields are rarely just blown out during harvesting. It was found that spores strongly adhere to open bolls and survive through the harvesting process to worsen the colour grade of the harvested lint. However, one or twocolour grades may be restored through the ginning process. Between 30-50% of heavily contaminated sooty mould cotton in the total pick can drop colour grade from base grade to incur penalties. Colour grade, while not showing effects on lint or yarn quality in these experiments, was strongly related to dyeability of fabrics. Cotton colour degraded through weathering and rainfall affected dyeability more that colour degraded through sooty mould fungi and this was most likely associated with the lower moisture levels received by the sooty mould cotton. SEM scans did not show obvious surface changes due to sooty mould activity.

At this stage options for mitigation of the degradation of colour grade are not available. There is no mitigation for rainfall and growers already aim to harvest their crop as timely as possible to avoid weathering and colour degradation. Agronomic and management practices are being investigated in a continuing project. As colour degradation due to sooty mould development largely depends on the presence of sugars in the form of insect honeydew, the most important goal a grower should have is to avoid insect pests such as whiteflies, aphids and mealybugs. This can be achieved through sound, IPM guided crop management using IPM compatible insecticides to control early season pests such as mirids in order to preserve beneficials that later on help with management of Silver leaf whitefly (SLW). Should conditions conspire, and a crop is sticky without rainfall in sight, a range of scenarios may be managed with suitable knockdown insecticides which may reduce SLW numbers until defoliation is complete. On the other hand, if rainfall is in sight, sticky cotton can benefit from this but incurs the risk of sooty mould development. Fungicides tested in this project have the potential to prevent sooty mould development, however, none are currently registered for use on cotton (other than as seed treatment). If registration becomes an option in the future, many parameters would have to be considered including environmental impact, pre-harvest withholding periods, inherent effects on lint, application efficiency and cost. Further research into mitigation options are being addressed in CSP 1901“Reducing the impact of weather, insects and microbes on cotton colour”, a collaboration between Dr Simone Heimoana (Simone.Heimoana@csiro.au) and Dr Stuart Gordon (Stuart.Gordon@csiro.au). Management of honeydew related cotton discolouration is up to the individual grower and while sooty mould tends to affect the individual grower with colour discounts, sticky cotton has the potential to penalise the entire industry.