Gray mold and Rhizopus rot on strawberry in field.
The gray mold (Botrytis cinerea) and Rhizopus rot (Rhizopus stolonifer) are the main causes of strawberry post-harvest decay. The infection of the fruit by gray mold can start in the field by flowers, B. cynerea remains latent underneath the sepals until the fruit ripening, and then after harvest it can turn from a saprophyte into a parasite. The fungus can also develop at low temperature (even at 0°C), with the consequent shortening of the period of storage and marketing.
Rhizopus rot (on left) grigia and gray mold (on right) during postharvest storage.
Rhizopus can develop at temperatures greater than 4-6 °C, and it is more common on fruit in the field when high humidity conditions occur (rains in open field or high humidity under plastic tunnels). Furthermore, infections from Penicillium spp. (blue mold) and Mucor spp. (Mucor rot) also occur occasionally.
Gray mold and Rhizopus rot on the same fruit.
In conventional agriculture, these infections are controlled by fungicide treatments that are applied close to flowering and repeated up to the harvest, however the synthetic fungicide application is not allowed either in organic agriculture or in post-harvest storage, thus in these years several research teams have studied alternatives to the use of synthetic fungicides, especially the studies are focused on natural compounds and resistance inducers and their potential for large-scale application. Resistance inducers can increase plant defense, and at times can also exploit their antimicrobial properties.
Disease starts in field where Botrytis cinerea remains latent underneath the sepals until the strawberry ripening.
A research team of Università Politecnica of Marche Italian Region (Romanazzi et al., 2013) has evaluated the efficacy of both chitosan and other resistance inducers in controlling the post-harvest fungal decay of strawberry.
For the study, the researchers have at first compared the following different acid solutions with practical grade chitosan:
- Chitosan dissolved in acetic acid;
- Chitosan dissolved in glutamic acid;
- Chitosan dissolved in formic acid;
- Chitosan dissolved in hydrochloric acid;
- Water-soluble commercial chitosan formulation;
- Control (strawberries untreated).
- Commercial chitosan formulation;
- Soybean lecithin;
- Calcium and organic acids;
- Extract of Abies sibirica;
- Extract of Urtica dioica;
The fruits used on the harvest day, were immersed for 10 seconds in a 5 L volume of the different solutions, then the fruits were air dried for 1 h and then put in a small plastic box. Packaged fruit is stored for 7 days at 0±1 °C and 95-98% RH, and then exposed to 3 days shelf-life at 20±1 °C and 95-98% RH to simulate retail conditions.
Fruit treated with water-soluble commercial chitosan formulation (on the right) and control (on the left).. Click here to enlarge.
During the storage, the researchers evaluated the percentage of decayed strawberries by using an empirical scale with 6 degrees:
- 0, healthy fruit;
- 1, 1-20% fruit surface infected;
- 2, 21-40% fruit surface infected;
- 3, 41-60% fruit surface infected;
- 4, 61-80% fruit surface infected;
- 5, more than 81% fruit surface infected and showing sporulation.
However, the practical grade chitosan dissolved in acid solution and the commercial chitosan formulation differ profoundly in their preparation technique. The chitosan dissolved in acid solution must be prepared 2 days before application to monitor and adjust the solution pH, while the commercial chitosan formulation can be prepared 2 hours before application, just by dissolving the powder in water. For this reason, the water-soluble commercial chitosan formulation, being applied more easily in the field, satisfies better the feasibility request of growers.
The second part of the study on effectiveness evaluation of resistance inducers showed that all experimental natural compounds tested reduced significantly the infections of gray mold, Rhizopus rot, and blue mold on stored strawberries. In general, commercial chitosan, benzothiadiazole, and calcium with organic acids resulted to be the most effective by reducing the gray mold by 79%, 73%, and 70%, and the blue mold by 90%, 84%, 71%, respectively. In addition, the commercial chitosan formulation and calcium with organic acids resulted effective in control Rhizopus rot by reducing the infection by 84% and 79%, respectively. Thus, the last two treatments resulted the most effective with the greatest action range.
The researchers concluded highlighting the importance to continue the study testing the same compounds, which resulted effective in the control of storage decay of strawberry, ever since in the field during flowering and a few days before harvesting to enhance the effectiveness of these natural compounds against fungal infections.
The research was conducted within the European Project EUBERRY (www.euberry.univpm.it).
Original research. Romanazzi G., Feliziani E., Santini M., Landi L., "Effectiveness of postharvest treatment with chitosan and other resistance inducers in the control of storage decay of strawberry", 2013, Postharvest Biology and Technology, Issue N. 75, pagg. 24-27. Further info: http://www.sciencedirect.com/science/article/pii/S0925521412001809