Internal fruit rot of capsicum (VG17012)

Internal rot of capsicums is a major ongoing issue for Australian growers. This disease, caused by fungi such as Alternaria and Fusarium, generally affects the seed and placenta inside the fruit rather than the edible portion. However, it can also occur on the blossom end of the fruit and, if left unchecked, will spread into the flesh. Although infection is believed to occur at flowering, the disease usually does not develop until the capsicum starts to ripen, with spread most rapid after harvest. As the disease cannot be detected from the outside, infected fruit can be sent to market. This can lead to affected consignments being downgraded or destroyed, as well as affecting consumer confidence and purchases. 

An infection model was developed by correlating internal rot occurrences in capsicums from various locations with local climatic conditions during flowering. Parameters such as total precipitation, air temperature, relative humidity, and wind speed were analyzed. The results indicate a strong negative correlation between wind speed and infection rates, particularly in the weeks during most flowering. This suggests that still air conditions may facilitate infection. In contrast, no consistent correlation was observed with plant wetness, humidity levels, or total rainfall. This observation suggests that spores may be able to germinate and infect flowers regardless of humidity or rainfall, potentially aided by the exudate from the flower stigma. Further study on the role of air movement and stigma exudate in the infection process, and an exploration of varying row spacing to enhance airflow, are recommended.

Recent trials completed in Bundaberg and Sydney on research and commercial properties examined the effect of different spray treatments on incidence of internal rot. In Bundaberg, capsicum plants were sprayed with Alternaria alternata spore suspension to attempt to increase infection, then a range of fungicides were applied throughout the growing season- particularly around flowing. No significant differences were found between treatments.

In Sydney, trials at two sites examined the effect of a wide range of fungicides, as well as biological products, on the incidence of internal rot. Products were applied using a conventional sprayer or an electric fogger, with the aim of increasing coverage on the undersides of the flowers. Results are still being analysed as the trials were only completed on 24 May. However, initial assessments suggest there are not likely to be significant differences between the control and treated fruit. 

The next stage of the project will be to attempt to further validate the model, as well as initiate further trials examining the effect of air movement on incidence of internal mould. These are expected to be conducted in Bowen and Bundaberg.

Internal mould of red capsicums is a major cause of consumer complaints, as there are no external symptoms that allow removal of affected fruit. The disease also cannot be reliably graded out using a near-infrared scanner, as a previous assessment showed it cannot distinguish between internal mould and the common physiological disorder dark seed.

The key environmental and cultural practices that favour growth of the fungal pathogens that cause internal mould are now being investigated. Replicated trials are underway in Bundaberg to evaluate the efficacy of targeted applications of agrichemicals and beneficial microbial products during flowering. A second trial in Bundaberg is evaluating the effect of a reduced spray volume on disease incidence at a commercial farm.

A third trial is planned on a farm in the Sydney Basin. This will test many of the same treatments being evaluated in Bundaberg. The site is located within a commercial farm and is located well away from the flood-prone site used for trials in 2021 to 2023. Environmental data from all sites is being correlated with disease incidence and severity.

Published disease forecasting models used for Alternaria diseases of other vegetable crops are being assessed to determine their applicability to capsicum internal mould. A greenhouse experiment is also commencing to determine if fruit infection only occurs at flowering or if the retained style at the fruit apex can act as an infection point during fruit maturation. This information is crucial as it governs the disease epidemiology and the success of different disease forecasting models and timing of chemical application.

Another knowledge gap required for developing a successful disease forecasting model is to determine the temperature relationship with growth rate and spore germination of the causal Alternaria and Fusarium species in in vitro experiments. Recent discussions with collaborators in Bundaberg investigated the possible deployment of a fungal spore trapping system with AI-control of spore recognition and an field weather station that has an in-built Alternaria disease forecasting model.

Project progress continues despite flooding and travel restrictions impacting project activities.

Modelling work in Bowen and Bundaberg has been moved to 2022 to verify whether models developed over the 2021-22 summer in NSW are widely applicable.

A new trial is underway at the Richmond lowlands site. Staggered plantings over a 12-week period are being used to model the relationship between the risk of internal mould and environmental conditions during flowering and fruit maturation.

A trial testing the effects of fungicides and chelated calcium is planned for the same site. A range of fungicide combinations will be tested, utilising different groups and modes of action. The effects of application with a normal backpack sprayer will be compared to a fogger, with the aim of increasing penetration into downward pointing flowers.

Fruit from these crops will be assessed to determine whether cold storage immediately after harvest can significantly reduce postharvest development of internal mould.

Since the last update, the project has progressed, with outputs from the initial, detailed literature review condensed into a format suitable for sharing with growers and supply chain members.

The team also isolated four fungal species from infected capsicum fruit, with three identified as Alternaria alternata, A. tenuissima and Fusarium oxysporum. Although the fourth species (possibly F. semitectum) was not re-isolated, this may have been due to a different spore type produced in culture. This species is now being re-tested.

Samples of brown and black seeds were also analysed, as they can appear to resemble the early stages of internal mould and be rejected retailers. With no fungi cultured from any sample, this is likely a physiological condition.

A trial was conducted to examine the effectiveness of NIR grading with capsicum fruit sourced from farms at Bowen, Gumlu and Bundaberg. The grader successfully detected and removed fruit with internal mould. However, dark / black seeds resulted in many otherwise good quality fruit being rejected.

Evidence from the grading trial suggested that cv. Shadowfax had a low rate of internal mould and fewer fruit with dark seeds compared with other varieties. Growers and wholesalers have also suggested that pointed, rather than blocky shaped, fruit are far less susceptible to internal mould. Variety trials have been planted at Stanthorpe and Richmond NSW to examine varietal differences to susceptibility for internal mould.

The project’s activities were presented at a well-attended webinar on 2 September 2020. With a high level of industry interest, a second webinar is planned. Variety trial results will be published once available.

Initial work has focused on isolating and identifying the organism(s) responsible for causing the disease. Infected flowers and fruit have been collected from around Sydney as well as Warwick and Bundaberg in Queensland. A number of different Fusarium and Alternaria species have been isolated and cultured.

Four of the strains found so far are now being inoculated onto flowers of four different varieties of capsicum plants. The fruit will be allowed to develop to full (red) maturity, then assessed for presence of internal rots.

ACT NOW

• Read article Exploring the internal issues affecting capsicums which details some early results from the project, found in the Winter 2020 edition of the Vegetables Australia magazine, p 58-59.
• Learn more about the internal rot of capsicums in the Autumn edition of the Soilless Australia magazine p 9, published by Protected Cropping Association (PCA).