Managing post-harvest rots in vegetables

Chris Galbraith<galbra53@msu.edu>, Michigan State University Extension - July 18, 2024

Implement best practices during the growing season, at-harvest and post-harvest for reducing spoilage losses during storage.

Cabbage beginning to spoil. — Photo courtesy of MSU Plant and Pest Diagnostics.

Post-harvest rots occurring in vegetable crops can deal a brutal blow both financially and in terms of morale. All the investment and hard work of growing produce is lost if the crop spoils before it can be sold. Fortunately, many of the factors influencing prevalence of post-harvest rots are well-known and there are actions growers can take during the growing season, at-harvest, and post-harvest to reduce losses from premature decay.

What causes a rot?

Once vegetable crops are harvested, they are cut off from their source of nutrients and water. However, the produce continues to respire, using up its energy reserves and releasing water during metabolism. Cellular breakdown occurs as the produce spends its energy/moisture savings without any means of replenishment. As the produce degrades in quality, it becomes more susceptible to colonization by rot-causing pathogens.

There are many different microbes that bring about premature decay if they are able to infect vegetable crops during the different stages of growth and storage. These organisms may be latent pathogens, which can remain quiescent upon entering the crop and begin colonization of host plant tissue during storage (eg. Colletotrichum). Pathogens may also begin colonization immediately after infection, in which case spoilage occurs rapidly. Many rot-causing microbes are considered weak pathogens (eg. different bacterial soft rots), which cannot penetrate healthy plant tissue but instead require wound sites (nicks, scrapes, bruises) to enter and begin colonization. Table 1 lists common post-harvest pathogens for various vegetable crops. Many of these pathogens not only occur in storage but can also cause disease during the growing season For assistance identifying a post-harvest pathogen, samples can be sent into the MSU Plant & Pest Diagnostic lab on campus. For more information, visit the PPD website.

Vegetable	Post-Harvest Pathogens	Symptoms
Pumpkins/Hard Squash	*Fusarium Rot (Fusarium* spp.)**	· Tan, corky lesions · White mold
	*Sclerotium Rot (Sclerotinia sclerotiorum)*	· White, fluffy mold · Hard, black pellets (sclerotia)
	*Cottony Leak (Pythium* spp.)**	· Brownish water-soaked lesions · White, cottony mold
	*Anthracnose (Colletotrichum* spp.)**	· Round, tan, black lesions · Water-soaked tissue around lesions
	*Black Rot (Didymella bryoniae)*	· Water-soaked lesions · Small black spots
Carrots	Gray Mold (Botrytis cinerea)	· Gray velvety mold
	*Watery Rot (Sclerotinia minor)*	· White fluffy mold · Hard, black pellets (sclerotia)
	Bacterial Soft Rot (various spp.)	· Sunken, water-soaked spots · Seepage from affected area
Potatoes	Bacterial Soft Rot (various spp.)	· Sunken, water-soaked spots · Seepage from affected area
	Dry Rot (Fusarium spp.)	· Sunken, brown/black areas of decay · White mold may be present
	Pink Rot (Phytophthora erythroseptica)	· Brown, dry sunken lesions · Brown internal rot
	Pythium Leak (Pythium spp.)	· Brownish water-soaked lesions · White, cottony mold
Onions	Bacterial Soft Rot (various spp.)	· Sunken, water-soaked spots · Seepage from affected area
	Black Mold (Aspergillus niger)	· Dark mold growth · Shriveling of bulb
	Blue Mold (Penicillium spp.)	· Soft, watery lesions · Bluish-green mold growth
	Basal Rot (Fusarium spp.)	· Red-brown rot at basal end
	Neck Rot (Botrytis spp.)	· Grey, velvety mold
Cabbage	Bacterial Soft Rots (various spp.)	· Sunken, water-soaked spots · Seepage from affected area
	Watery Soft Rot (Sclerotinia spp.)	· White fluffy mold · Hard, black pellets (sclerotia)
	Gray Mold (Botrytis cinerea)	· Grey, velvety mold
	Alternaria Leaf Spot (Alternaria brassicola)	· Small, brown lesions that turn black · Lesions are circular and spongy

Table 1: Common Post-Harvest Pathogens of Vegetable Storage Crops

The disease triangle is a useful framework. Spoilage occurs if the pathogen encounters a susceptible host in conditions that are favorable for infection. Post-harvest spoilage occurs more frequently in warm, wet and humid conditions. High temperatures encourage pathogen activity and increase the rates of produce respiration/ripening. Excess moisture on produce encourages pathogen activity and entry into plant tissue. Similarly, high relative humidity supports free moisture accumulation on vegetable crops in storage. Controlling these factors mitigates risk of premature spoilage and increases the shelf life of vegetable crops.

Disease Triangle.jpg — The disease triangle is a useful framework - spoilage occurs if the pathogen encounters a susceptible host in conditions that are favorable for infection.

Best Management Practices

Steps can be taken throughout production and storage to minimize losses from post-harvest rots. During the growing season, preventing fruit from coming into contact with the ground using plastic or organic mulch keeps produce cleaner and reduces opportunities for infection. Mulch also helps to reduce soil splash during irrigation or rainfall events, further limiting pathogens from being moved onto susceptible tissue.

Minimizing damage to produce throughout the season reduces wound sites and therefore pathogen entry points. This means gentle handling of vegetable crops at harvest and reducing the number of times produce is touched to minimize accidental skin breaks or bruises. Protecting vegetables from insects, wildlife, and environmental damage (hail, sunscald, wind) also reduces wound sites. Culling produce that is already damaged can reduce spread of spoilage in storage bins. Harvesting at proper maturity also helps optimize shelf-life.

Keeping vegetables cool can go a long way toward preventing premature spoilage. Harvesting during the coolest part of the day and removing the field heat from vegetables immediately after harvest is key. There are different pre-cooling methods available to rapidly bring down the temperature of produce prior to storage, including room-cooling, forced-air cooling, hydrocooling, package icing and vacuum cooling procedures.

Storage temperature and humidity recommendations vary by vegetable crop and following the right guidelines is important for maximizing shelf life. Specific storage recommendations and information on pre-cooling methods can be found in the Guide to Vegetable Production in Ontario (OMAFRA Publication 839). Maintaining proper temperature is also important to prevent chilling or freezing injury, which presents as sunken, water-soaked spots on susceptible produce.

For certain storage crops, curing is a crucial step for optimizing shelf life. Curing allows skins to dry out and harden, increasing protection against infections that cause rot. The area used for curing should always be a well-ventilated space with adequate air circulation. Recommended curing guidelines for several storage vegetable crops are listed in Table 2.

Vegetable	Time	Temperature	Relative Humidity	Source(s)
Hard Squash (Hubbard, Buttercup, Kabocha, Butternut)	5-10 days	80-85°F	80-85%	Pumpkin and Winter Squash Harvest and Storage, University of Massachusetts Extension Winter Squash Curing & Storage Chart, Johnnys Selected Seeds
Onions	2-4 weeks	75-80°F	70-80%	2024 Midwest Vegetable Production Guide Growing onions in home gardens, University of Minnesota Extension
Garlic	Up to 2 weeks	75-90°F	60-75%	Organic Garlic Production, Michigan State University Extension
Potatoes	10-14 days	50-60°F	95%	Potato Harvest and Storage, University of Massachusetts Extension
Sweet Potatoes	3-5 days	85°F	85-90%	Guide to Vegetable Production in Ontario (OMAFRA Publication 839)

Table 2: Curing Guidelines for Storage Vegetable Crops

In conclusion, there are various factors that contribute to the occurrence of post-harvest rots. By taking the right action during the growing season, at-harvest, and post-harvest, losses from premature decay can be minimized and crop quality preserved. Please reach out to your MSU Extension vegetable educator in your region if you have any questions.

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