The impacts of rainfall and sooty mould contamination on cotton colour grade

by Simone Heimoana and the team in Narrabri, with thanks to Andrew Baxter (Australian Classing Services).

Cotton lint colour is an important quality parameter that can be affected by a number of factors including weather conditions, boll maturity, insect activity leading to sticky honeydew deposition and microbial agents. Interactions between these factors are typical, especially when climatic and in-field conditions enable fungal growth on cotton lint that has a high sugar content due to immaturity or insect honeydew. The main agent of lint discolouration in the field is rainfall, in particular exposure to heavy or prolonged rain events that negatively affect lint colour and cause downgrades. The lint of newly opened mature cotton bolls is white, clean and lustrous because cellulose, its primary component, is highly reflective. The cotton fibre is also covered by a waxy cuticle that helps to repel moisture but it does not make the open boll impermeable to water. Colour grade is determined by a combination of greyness, which indicates how bright or dull cotton is, and yellowness, which indicates the degree of colour pigmentation. The combined measurements are expressed as a colour grade ranging from 11 (high quality) to 61 (low quality), with the Australian base grade at 31 (Middling).

The two questions that we wanted to answer in the Cotton Research and Development Corporation (CRDC) supported project (CSP1703) were:

  1. How much rainfall is required to reduce lint colour by one grade? and
  2. What proportion of the crop needs to be contaminated with sooty mould before a colour downgrade can be detected?

We were able to answer the first question during an unusually wet harvest during which open bolls were exposed to the effects of a cumulative rainfall total of 202mm over 21.5 weeks. Control bolls were sheltered by a plastic greenhouse while exposed bolls from the lower crop canopy were harvested after each significant rainfall event. Cotton colour from each harvest was assessed. Exposure to 20mm of rainfall, which fell in 4 separate events of less than 8 mm did not affect colour grade.  However, from May onwards, rainfall events were accompanied by cloud cover and the crop could not dry out. After each of those periods, colour grade dropped. Colour grade dropped from base grade 31 to 41 after being exposed to 141mm over 15 weeks.

The effect of field exposure to cumulative rainfall on cotton colour grades, Australian Cotton Research Institute, 2016. Control bolls (0 mm) in the left column. RF indicates rainfall amount.


To answer question two, we set the upper limit of sooty mould impact on lint colour by selecting bolls that were intentionally contaminated with heavy levels of mould and mixing these in different ratios with clean cotton. In a field situation, however, there would be a gradient of contamination, and spatial variation within the field, which would lessen the impact on colour quality. We have not tried to address this here.

In our experiment we generated heavy sooty mould contamination by spraying a sugar solution over open boll cotton plants in field tents. The tents were closed up and over several days sooty mould grew, aided by the warm and humid environment inside. We then mixed the heavily contaminated moldy cotton bolls with clean bolls in varying proportions ranging from 100% clean to 100% moldy and ginned them. The initial measurement of the clean seed cotton was 21-1 to 31-2. The 100% clean cotton sample with a base grade of 21-1 to 31-2 improved to 11-1 after ginning, indicating that the inner boll lint was in better condition than the lint on the outside of the boll. Once the proportion of contaminated lint reached 20%, grades dropped to 31-1 and 31-2. However, with between 30-50% of bolls contaminated, there was a downgrade to 41-1 and 41-2, which would result in quality discounts, and past 60% contamination, grades fell to 51-1 and 61-1, which would result in even heavier discounts.

While whiteflies and aphids occur only sporadically in Australian cotton fields, there may be years where pest management is difficult and affected by influxes from other areas, dense canopies, and unfavorable weather. It is in this situation where heavy mould contamination in the lower third of the crop could be problematic, and may lead to colour downgrades if heavily contaminated bolls make up 20% or more of the harvest. However, not all infestations of pests will lead to heavy contamination.

Clean and sooty mould contaminated cotton mixed in different ratios (from 100% clean, 100:0 to 100% contaminated 0:100) to evaluate final lint colour grade (black label on bottom photo). Base colour grade of seed cotton indicated in red.


Link to final report on this topic CSP1703.