We have all heard that one bad apple will spoil the bunch. Which is true. But, then the question arises as to how the first apple went bad to begin with. There has to be a starting point for the initial pathogen infection, which results in the development of vegetable and fruit diseases. (See the previous post on pathogens and microorganisms to see how they can affect produce.) I have outlined some of the most common methods of how fruit and vegetables become infected with diseases below.
Table of Contents
Latent Infections
Note: There are infections that are known as latent infections, or quiescent infections, that occur but do not appear at the time of infection. They are essentially dormant for a period of time, which is why produce can be infected prior to harvest and issues do not appear until after harvest or when storage conditions are altered. The period of latency eventually ends and pathogen development becomes visible. This typically occurs when produce is stored in non-optimal or “stressful” conditions, which is normally beneficial for pathogen growth.
Pre-Harvest Infections
- Flower Development – There are certain pathogens that infect the flower during development, but do not appear until after the produce is harvested.
- Irrigation Water – Produce that is infected with irrigation water prior to harvest can demonstrate symptoms before or after harvest. If the symptoms appear before harvest, the produce is not sent to be packed. If the symptoms do not appear before harvest, they will either appear during storage, or any step thereafter.
Infections at Harvest
- Wounding – This includes everything from stem punctures, where the stem of one fruit punctures the skin of another piece of fruit, to fingernail wounds, to any form of wounding that results in bruising. Once there is any form of opening in the produce, that allows for pathogens to enter. When this occurs, produce is especially susceptible to airborne pathogens that can enter the wounds.
Postharvest Infections
- Wounding – Just as wounding can affect produce during harvest, it can have the same effect postharvest. However, during the postharvest handling process, produce items are exposed to additional methods of contamination. For example, if the fruits or vegetables are being washed and packed in a facility, they are exposed to any pathogens that could be located in the wash water, as well as additional handlers that are doing the packing that can inadvertently lead to wounding.
- Infected Produce Items – This is where the bad apple affecting the rest of the apples comes into play. However, it is not just fruit affecting others in the same container. It can also be where different produce items are stored, transported, or displayed near each other and the pathogens can transfer onto different produce items. A recent example I encountered was when I was buying apricots in bulk at Costco. They are sold in clamshell trays, with each apricot having its own individual tray. When I looked through the first tray, there was one apricot with mold growing on it. The second tray had two pieces of fruit infected with the same mold, and then as I kept looking, all of the trays had at least one apricot infected. This is the initial display of the transfer of pathogens, and demonstrates that the fruit does not need to be touching each other in order for the infection to spread.
How to Prevent Pathogen Growth & Spread at Home on Fresh Produce
Just like pathogen transfer can happen before the produce gets to you, it can just as easily, if not more easily, occur once the produce gets to your house. For example, when produce is packed and cooled before arriving at the grocery store, it is kept at optimum temperature. However, once it is on display and in your house, it is no longer at that optimum temperature. Even if you put the produce that should go into the fridge in the fridge, no one’s fridge is really set at 0°C or 32°F, so that could be the temperature where the latent pathogens can start to grow.
Some methods to prevent pathogen growth and spread at home are:
- When buying in bulk (e.g., bags of cherries or grapes), make sure you inspect the bag at the grocery store before buying them. If you find an infected piece of fruit, the fruit around it has most likely already been infected as well. Also, once you get home, you should do an additional, more thorough, check to make sure that if any pieces of fruit are infected, that those and the fruit surrounding them are removed.
- Make sure you store produce properly once you bring it home. If you are supposed to put it in the fridge, put it in the fridge!
- If you are keeping fruit out on the counter to ripen, make sure that you check it periodically. If left unnoticed and any of those fruit items develop pathogen growth, it can quickly spread to the other fruit. I see this problem most often with avocados.
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- What is Food Safety & Why is it Important?
- How to Know When Cantaloupes are Ripe
Resources:
“Origins of Postharvest Diseases.” Washington State University. Web. Accessed 4 July 2018.
Rohrback, K.G. “Unusual Tropical Fruit Diseases with Extended Latent Periods.” Plant Disease July 1988; 607-609.
How about the biochemical changes after infection?