The effects of high day and night temperatures on fruit development

25 March, 2019

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The 2018 -2019 summer has been described as one of the hottest summers on record. High day temperatures, when combined with high night temperatures can affect plant transpiration processes and fibre production, and therefore yields. Fibre quality, namely micronaire, can be affected because of the loss of later fruit that would normally help dilute the micronaire range across the whole plant.

Ideal temperatures for cotton production are maximums ranging from 27°C to 32°C and minimums of 16°C to 20°C. Temperatures in excess of 32°C can cause the plant to transpire more water to keep cool. Night time temperatures above 22°C will affect transpiration. Temperatures below 11°C and above 36°C will cause plant shock and will affect development (Constable and Shaw 1988).

Table 1 (click image to see full table) shows that this season Day Degree accumulation for the specified period is almost 21% higher than the long term average. This is from 62 years of data.

Cold shock days (<= 11°C) are significantly down on the long term average and Hot shock days (>=36°C) are above the long term average.

Effects on fruit development
High temperatures and high humidity can cause damage to plant tissue. Day temperatures above 35°C and night temperatures above 26°C mean that leaf temperature and respiration rates remain high. This will reduce the amount of assimilates for fruit production and fruit shedding may occur.

‘Parrot beaking’ of bolls is caused by high temperatures at flowering and pollination. These bolls have uneven seed numbers in one or more locules. High temperatures reduce the viability of pollen. With reduced boll size and the full opening ability (of parrot beaked bolls) yields may well be affected.

‘Boll cavitation’ is where the boll dies before the abscission layer is formed. The boll will ‘dangle’ from the fruiting branch and be visible in the crop. This is different from physiological shedding where a square or small boll will form a complete abscission layer and fall to the ground.

This loss of fruit may cause the plant to want to grow excessively and careful management with both a plant growth regulator and irrigation timing and intervals are required.

Further reading: CottonInfo, January 2018: ‘Managing heat stress in cotton’: Dr Mike Bange and Dr Rose Brodrick

SOS Macquarie update
The number of spray drift issues has dramatically reduced during the 2018-19 cotton season in the Macquarie Valley and anecdotally across the whole cotton belt. This is a function of a very hot and dry season, more education about the risks of spraying in inversion conditions and new 2,4-D label and nozzle requirements. There was a lot of fallow spraying up until Christmas and mid-January. It is encouraging that the number of investigations has decreased and it augers well for the future.

SOS Macquarie recently conducted a series of spray application workshops that were very well attended. These workshops are part of an ongoing plan by the group to help eliminate spray drift. Other cotton regions are looking at the SOS Macquarie model to potentially launch in other areas.

By Craig McDonald, CSD Extension & Development Agronomist – Central NSW