| In 2003 several greenhouse growers reported infection of pepper fruits showing a fungus inside the fruit without showing any obvious outside symptoms rendering the infected fruits un-saleable and unusable. Growers tried to screen the fruit based on external observation but were not successful and a significant amount of fruit had to be destroyed. The disease became a serious marketing issue. Therefore, a research project was launched by scientists at the Alberta Research Council (ARC) and Alberta Agriculture in 2004 to determine the cause of the disease, how it psreads, and how to control it. Financial supports were obtained from ACIDF, AGGA, ARC and Red Hat Coop. This brief report gives the results achieved so far.
Main objectives of the study were to determine:
- The cause of the disease
- Are there any varieties resistant to the causal fungus
- How the fungus spreads in a greenhouse
- How serious is the disease in Alberta greenhouse
- How to control and avoid the disease
The Cause:
Infected red, yellow and orange pepper fruits and stems were collected from commercial greenhouses in Alberta and processed for fungal isolation. More than 40 Fusarium isolates were recovered; these isolates mainly belong to two species, Fusarium solani and Fusarium proliferarum.
Fusarium solani caused rapid infection and showed external fruit and stem infections in 14 days, while Fusarium proliferatum caused internal infection of inoculated fruits and developed slowly and showed symptoms in about 40 days after inoculation.
It was observed that orange pepper (‘Sympathy’) had the greatest amount of internal fruit rot while the red variety had less internal infection but had most outer infection which started at the blossum and/or penduncle end (Figure 1). Stem infection become severe in June and July.
A similar disease, caused by Fusarium subglutinans was reported in B. C. in 2002. However, the pepper fruit disease in Alberta exhibiting internal fruit symptoms is a new disease and is caused by Fusarium proliferatum.
Disease survey:
In the Fall of 2004, extensive disease survey was conducted in Alberta. Special emphasis was placed on detecting internal fruit rot, the new disease. Eight commercial greenhouses and one experimental greenhouse growing sweet peppers were surveyed. Information on cultural practices, disease history and disease management practices was gathered from growers. Approximately 25% of the plants in each greenhouse were visually examined for symptoms of stem and fruit rot diseases that were present. Samples of diseased plants and fruits were collected and taken to the laboratory for microscopic observation, pathogen isolation and/or tissue testing. Samples of raw water, as well as feed, leachate and/or return (recirculation) solution, were taken for pathogen testing. Structural components, walls, walkways, work tables, pipes etc that could harbour pathogens were checked for the presence of fungus. Petri plates of potato dextrose agar were exposed to the air in various areas of each greenhouse for one minute to assess the airborne microbial load. Mature fruits were randomly picked up, cut open and checked for internal infection.
Results showed that the major causal agent of internal fruit rot on pepper in Alberta greenhouses was F. proliferatum that has never been reported in greenhouse peppers in Canada. Internal fruit rot was about 3 percent on orange pepper. Other serious disease was Fusarium stem rot which ranged from 5 percent (yellow pepper) to 12 percent (orange pepper).
A detailed brochure on disease occurrence, description and damage has been prepared and distributed to greenhouse growers for education purpose.
Disease spread in greenhouses:
Spore trapping using a special spore trap and culturing techniques is in progress in several commercial greenhouses in Alberta. Results obtained so far indicate that Fusarium spores are present in the air in infested greenhouses. Pollinator bees, captured and processed, were found to carry Fusarium spores on their mouth parts and legs. Further studies are in progress
Other studies in progress:
To develop biocontrol of the disease, we found 8 bacterial isolates that showed certain level of inhibition of Fusarium isolates. Further screening to develop a biocontrol agent(s) is underway.
Experiment is in progress in Vegreville to determine if there are any varieties resistant to the disease. Based on the tests so far, white and orange peppers were more susceptible to the pathogen than red, brown and yellow peppers. The study will be completed in the next few weeks.
Attempts are also being made to determine if the fungus Fusarium proliferatum produces any mycotoxin in the infected fruits. The toxin, Fumonisin B1, was detected in infected pepper fruit tissues at a concentration of as high as 145ppm. Further tests to confirm the results and to determine the influence of environmental conditions on toxin production are being planned.
Preliminary work on DNA-PCR assay is being carried out to classify the Fusarium isolates correctly and to develop molecular diagnostic tool.
Future Studies Planned:
DNA sequencing can be done to classify the Fusarium isolates correctly and to develop molecular diagnostic tool. This information will be helpful in developing screening technique for disease resistance.
Disease process needs to be studied to reveal insight of the disease development. Information on how the disease started, how it enters the fruit, source of primary inoculum, the disease cycle and the host range of the pathogen are needed to fully understand the relationship of the pathogen and the host, and further to develop disease control methods.
Epidemiology study needs to be done to determine effects of environmental factors, such as temperature and relative humidity, on the disease development. Information will help to develop integrated greenhouse management strategies to eliminate the disease.
Figure 1. Internal infection of a red pepper fruit (“Early California Wonder’, a and b) and a orange pepper fruit (‘Sympathy’, c and d) inoculated with Fusarium proliferatum in a greenhouse in Vegreville. The mature fruit appeared healthy externally (a and c), but was infected internally (b and d).
Acknowledgements: A cash grant from Red-Hat Co-op of $4,000 and in kind contribution from Pik-N-Pak is gratefully acknowledged. Assistance of Alberta Greenhouse Growers Association in supporting this research is appreciated. |