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Thermofogging fungicides into coolrooms

Pest and Disease Management

Dr Gordon Brown reviews the new technology of thermofogging of cold storage facility systems for the control of post-harvest rots. This article appears in the Winter 2022 edition of AFG.

Take home messages

Thermofogging:

  • effectively controls post-harvest rots and uses an unusual fungicide group, which is good from a disease resistance management perspective
  • allows for rapid cooling of dry fruit
  • maintains a dry floor around the storage facility, with less risk of accidents
  • eliminates the need to dispose of leftover fungicide dips
  • can be used to apply DPA and is compatible with 1-MCP.

 

Modern consumers have become very concerned about the environment and the food that they consume. This is putting pressure on growers to reduce the total quantity of pesticides that they use and accidentally release into the environment. Both synthetic and organic fungicides can accumulate in the soil and groundwater, and this should be avoided.

One way of reducing the total quantity of fungicide used is to replace field applications with post-harvest treatments for the control of storage rots. Traditionally, post-harvest applications have involved drenching or dipping freshly harvested bulk bins of apples to thoroughly wet all fruit prior to placement in the coolstore. This activity requires the use of specialised drenching equipment, drench monitoring and maintenance. Further, the extra handling steps slows down the placement of the bins in the cold room and the start of the fruit cooling process.

 

Figure 1: Penicillium rot of a stored apple.

 

An alternative application strategy is the use of a new technology to apply the fungicide within the coolstore atmosphere. This reduces the double or triple handling of the bulk bins to a single step and allows for rapid cooling of the fruit after harvest to assist in maintaining fruit quality. Recently, a new technology – thermofogging of cold storage facility systems for the control of post-harvest rots – has become available in Australia (ecoFog® and ActiMist™). This equipment can also be used to apply DPA if 1-MCP has not been used. The active ingredient currently used with fungicide thermofogging is pyrimethanil – a broad-spectrum fungicide belonging to Group 9. The only other Group 9 fungicide in Australia is cyprodinil, which is allowed to be used on apples at flowering. Hence pyrimethanil fits in well from a disease resistance management perspective. Here we will review research conducted in Germany on stored apple fruit quality, using the same active ingredient and application technology as is available in Australia (Büchele F et al. 2022).

Method

In this German research, the Xedathane_HN (156g pyrimethanil/L) product was used at a rate of 50ml/t of apples. This is similar to the Australian products (30 to 60ml/tonne of a 160g/L product). The trees in the experimental orchard were sprayed 20 times with fungicide cover sprays leading up to harvest. Sprays were terminated 5 weeks prior to harvest and a small group of trees received an additional four cover sprays (Captan and Trifloxystrobin) to act as a spray control. This spray program is reported to be typical for Southern Germany. Unfortunately there was not a treatment that included a post-harvest dipping of the fruit. Fruit were harvested and half the “unsprayed” trees fogged with pyrimethanil. Apples were stored for 5 months at 3°C in air storage, and then rotted fruit were assessed and removed, and the healthy fruit placed at 20°C for 2 weeks prior to a final fruit rot assessment.

Results

Figure 2: Rotted apples with different fungicide treatments when removed from cold storage, additional fruit losses after 2 weeks at room temperature and the total percentage of crop lost to fruit storage rots.

In this trial the dominant post-harvest fruit rot was caused by Neofabraea species; these rots are commonly called bitter rot, bull’s eye rot or target spot, and are common causes of fruit storage rots in Australia. There was also a small number of fruit with Penicillium fruit rots – also a problem in Australia. Apples that had not received cover sprays up to harvest nor thermofogging had severe levels of fruit rots both at the time of removal from cold storage and after 2 weeks at room temperature (Figure 2). Combined, over 25 per cent of stored fruit in this treatment was lost to fruit rotting and, importantly, a large number of these fruit would have developed fruit rots during transport and marketing. Both the thermofogged fruit and those that had received cover sprays up to harvest had minimal fruit rots when they were removed from cold storage. After two weeks of storage at room temperature, however, the thermofogged fruit had developed slightly fewer fruit rots than those that had only received cover sprays up to harvest. These results clearly show the efficacy of thermofogging for the control of post-harvest fruit rots.

Consequences

This research has shown that thermofogging fruit with pyrimethanil is an effective method of controlling Neofabraea fruit rots, both during storage and subsequently during transport and marketing at room temperature. In total, for a 50t/ha crop, 400g of active ingredient was used in the thermofogging treatment compared to over 4kg of active ingredient in the cover spray, so thermofogging has a lower environmental impact. While the quantity of active material used when drenching fruit will be similar to the thermofogging method, thermofogging also presents a number of advantages: being able to store dry fruit; getting fruit quickly into the coolroom; maintaining a dry floor area; reducing bulk bin handling; and eliminating the need to dispose of leftover fungicide dip.

Further reading

Büchele F et al. (2022) “Assessment of a Postharvest Treatment with Pyrimethanil via Thermo-Nebulization in Controlling Storage Rots of Apples”, Agronomy

 

Acknowledgement: This article is provided under the National Apple and Pear Communications Project (AP21000) which is funded by Hort Innovation using the apple and pear R&D levy and funds from the Australian Government and is delivered by APAL.

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