Enhancing Fruit and Vegetable Quality for Small Farm Systems in the South
Penelope Perkins-Veazie, Research Plant Physiologist

Fresh fruits and vegetables are abundant in the United States and consumer appeal is necessary to make a sale. Unlike other commodities, such as grains or fibers, most fresh fruits and vegetables have a short shelf life (days or weeks rather than months), and have no consistent established marketing channels. If produce is to be exported, only the highest quality can withstand the rigors and abuses of transport and handling. Often, field practices initiated long before harvest can dramatically alter final product quality. Additionally, handling and storage practices at the time of harvest further determine the shelf life of the product.

Production Practices

Soil Type

Soil type will influence crop selection, tillage, irrigation, and plant disease and weed problems. Sandy ("light") soils drain well and warm quickly and are often used for early season crops or for crops that prefer a more dry environment. Sandy soils generally have low organic matter and low retention of water and plant nutrients, so must be irrigated and fertilized more frequently than heavy soils. Soils with more clay or silt in them are "heavier", warm more slowly, and retain more water than sandy soils. Heavier soils are used for warm-season crops or crops that have extensive and forceful root systems, such as watermelon and corn. Clay soils require careful management to avoid excessive water loss or compaction from over-tillage. Once clay soils dry to the point of cracking, application of water must be done slowly over a long period of time to allow penetration and rehydration and reduce erosion.

Tillage, Plant Spacing, and Cultivation

Production sites must be tilled before planting. Fruits and vegetables do not compete efficiently with cover crops or weeds and do not benefit from the no-till systems employed with some agronomic crops. Tillage aerates and 'fluffs' the soil, making it easier for plant roots to grow, as well as reducing plant competition. Additionally, beds 6 inches to 24 inches across, and 6 to 10 inches high, can be used to modify soils in less than ideal situations. For instance, brambles do not like wet roots but will establish well in heavy soils if planted on raised beds (better drainage).

Plant spacing is determined in two ways. First, the spacing of rows is determined, using measurements of the center of one row to the center of the next. Then, plant to plant spacing within a row is determined. Plant-to-plant and row-to-row spacing depends on the crop, the use, and the equipment available. For instance, U-pick strawberry farms often prefer raised beds with plants spaced in double rows one foot apart, and with 6 foot center-center spacing. This permits consumer accessibility to fruit with less bending. Brambles may be spaced from 6 to 12 feet center-center, depending on closeness of hedgerow pruning, plant vigor, and tractor size. Watermelon vines sprawl into aisles after a month or so of growth, limiting access to plant crowns, so spray or harvest alleys must be established in addition to rows. Cabbage destined for fresh markets should weigh 2 to 5 lbs when mature. To avoid heads that are too large, plants have to be spaced more closely, from 10 to 18 inches, depending on variety. The same variety of corn may not do well at the same spacing in different geographic areas. In some areas, corn will produce marketable ears when plants are spaced 6 inches apart while in other areas marketable ears will not develop unless plants are at least 12 inches apart.

Cultivation is used after the crop is planted to reduce weed growth and to add soil around the roots of potatoes and onions. Potatoes and onions tend to push the tuber and bulb portions out of the ground as the fibrous roots grow. The uncovered portions turn green when exposed to light and are considered unmarketable. In some perennial crops, such as blueberry and blackberry, use of a perennial grass in the centers helps prevent erosion and dust and competes effectively against weeds.

Irrigation

Timely application of water is one of the most important farming practices. Dependence on rainfall frequently results in too little or too much water at the wrong growth stage. The amount of water affects tissue size, flavor, and appearance. Too little water results in small tubers and fruits, such as potatoes and tomatoes or causes stress responses and nutritional disorders. Lack of available water affects the mobilization and uptake of several nutrients by slowing root hair growth, concentrating salts, and slowing nutrient mobility in plants. Nitrogen, calcium and boron are not taken up or moved in the plant, with reductions causing yellowing, blossom end rot (in squash, melons and tomatoes), internal browning (potato), tipburn (lettuce and cabbage), and brown bud or stem and tuber cracking (in broccoli, beet, turnip). Fibrousness in young okra pods and bitterness in many of the Brassica family (broccoli, kale) also comes from too little water.

Too much water causes other problems. Often, periods of very dry weather followed by heavy rains causes development of blossom end rot, probably because water is moved into the fruit faster than calcium. In fruits close to full ripeness, excessive water causes more uptake, with swelling, and sometimes cracking, resulting. Some fungi, especially soil-borne types, use rainfall as a means of dispersal and can cause epidemics of plant disease. Excessive water frequently means high relative humidity and favors the germination of fungal spores, and mycelial growth on plants. Spread of disease is further promoted by the increased thinness of plant tissue and occurrence of fruit cracks, that aid penetration of fungi. Fruit sweetness is often reduced with high rainfall as sugars are diluted by plant uptake of water. Excessive water leaches nitrogen from the soil, resulting in yellowing and reduced leaf or fruit growth.

Available water can be regulated in several ways. Application of a mulch to the soil can reduce the rate of water penetration as well as the rate of loss through evaporation and transpiration. Mulch materials such as bark, paper, plastic, or pine needles also reduce dispersal of soil-borne fungi. Irrigation practices through regular or monitored application of water reduce the chances of plant stress and a more even distribution of water during growth can help fruits cope with excessive rainfall. Alternatively, water application can be manipulated to improve the sweetness of some crops. Reducing irrigation in the last 2 weeks of melon fruit growth helps increase the sugar content yet has a minimal effect on melon size. Waiting until visible leaf wilt before applying water is not very effective as the plant has already experienced stress. Available soil moisture can be monitored by using irrometers, with irrigation applied before water moisture falls below a preselected value, usually about 50 kpa.

Nutrition/Fertility

Plants make their own food from light and carbon dioxide. However, the elements nitrogen, phosphorous, and potassium (N-P-K) are needed for efficient light harvesting, to complete the transformation of carbon dioxide to sugars, and for transport, storage, and utilization of sugars.

Several immediate postharvest problems, as well as the total shelf life, of fruits and vegetables are controlled by fertilizer type, quantity applied, available water, and soil acidity. Many of the sandy soils of the United States are acid (pH<6.0) and low in boron, calcium and magnesium and cause several defects in the harvested crop. This problem worsens in periods of drought because uptake of these minerals depends on root contact with soil water.

Nitrogen, potassium, and phosphorus are found in most general fertilizers. Fertilizers can be of inorganic (synthetic) or organic (composted manures, waste products) types. Usually synthetic fertilizers are more consistent in amounts of N-P-K than manures, but manures provide organic material to the soil. With either type, application of excessive nitrogen delays development of fruit and increases lush growth, increasing the chances for plant disease invasion. Lack of nitrogen can cause yellowing of fruits, such as cucumbers and okra, and small root crops, such as onions. Lack of potassium can result in fewer and smaller fruit. Lack of phosphorous can cause stunting of plants and reduce kernel fill in sweet corn.

Some minerals, although required by plants in amounts much less than for N-P-K, can be critical for productivity in certain crops, and are referred to as minor elements or micronutrients. Boron, a water-soluble element, is needed by Brassica crops and red beets to get a marketable crop, but can cause plant death in green beans if present in excess.

Pest Prevention and Intervention

In most small farm systems, integrated pest management (IPM) is practiced for both economical and environmental reasons. In the old days, prophylactic sprays were often recommended to prevent a problem from starting. However, increasing awareness that pests can adapt to survive chemicals, concern over water and soil residues, and the increased financial cost per spray have led to more selective use of chemicals, in combination with scouting and prediction systems.

Weeds. In the South, the weeds seem to grow an inch a minute. The heavy weed pressure from broadleaves and grasses can often best be reduced by effective application of an herbicide before planting the crop, and before weed emergence. After weed emergence, chemical choices are limited by the crop and harvest period. As with all chemicals, crop compatibility, label guidelines, and state laws must be observed.

Usually, weed control is desired more for crop establishment than for crop quality. Weeds indirectly affect quality by competing with crop plants for water and nutrients, and providing a habitat for insect pests.

Insects. All USDA grading standards require some degree of freedom (90-99%) from insect injury and presence of insects. Additionally, many countries require a phytosanitary statement to avoid importation of plant pests.

Most people object to finding half a worm in an apple. Besides the obvious turn-off of fauna in fruits and vegetables, insects leave residual feces (fras) that can be a host for mold growth. Although the chewed leaves of cabbage and the worm holes in fruits and roots are quite obvious cosmetic defects, other insects can cause more subtle damage. Stinkbugs feed on green tomatoes through a probe inserted into the fruit. These marks cause unripened dimples in red fruit. Thrips are tiny, flying insects that scrape the surface of leaves. The leaves of green onions with thrips damage have a whitish cast and a reduced shelf life because the wound causes increased water loss and allows entry of disease organisms. Heavy aphid or mite infestations reduces photosynthesis of melon plant leaves and reduces the sugar transported to the fruit, resulting in small and less sweet fruit.

Application of insecticides, whether used traditionally or organically, must be timed correctly and used for the appropriate insect. Insecticide labels vary among states in the crops used for; check with state extension personnel for crop use, application, and disposal guidelines.

Sometimes use of novel production practices effectively reduces the need for insecticides, such as planting crops to catch production windows where migrating insects are not present. Some mulches can affect insects by being too slippery to land on. Trap crops can be effective with some insects, such as planting yellow squash near watermelon or pumpkin to attract squash bugs. Weeds present in a cultivated crop often harbor insect pests. Additionally, biological controls that reduce harmful insects, such as predatory mites or wasps, or beneficial insects like lady beetles or lacewings, are now widely available.

Plant disease. USDA grading standards are quite strict on the appearance of plant pathogens in harvested fruits and vegetables. Once disease is visible on harvested produce, it can spread quickly and infect the entire load. Additionally, new research indicates that some human pathogens can enter diseased areas on fruit; produce with visible decay should therefore be discarded as soon as possible.

The invasion and spread of plant disease in harvested produce can be prevented to some extent at the preharvest level. Good field sanitation (removal of plant debris and fence row weeds, crop rotation, weed control) is one of the simplest and most effective ways of reducing the amount of pathogens present. Mulches can reduce dust and soil contact with leaves and fruit. Cultivar selection can be very important. Varieties are often developed for tolerance to diseases found in the particular area for which they are bred. For instance, cultivars with no resistance to leaf diseases can be grown in more arid parts of the U.S. but not in the more humid eastern U.S.

Although preventative sprays are used less frequently than in the past, routine fungicidal sprays can make the difference between a crop and no crop. For instance, brown rot (Monilinia fructicola) on peaches can take out the entire crop a week before harvest or can reduce the shelf life to a few days. Weekly protective fungicidal sprays, starting at bloom in periods of wet and cold temperatures, combined with orchard sanitation and insect control, are still needed for effective control.

While fewer organic options are effective with plant pathogens compared to insects, some states do provide leaf wetness data for various crops. These modeling systems predict the need to spray fungicides based on rainfall, temperature, changes in relative humidity in the microenvironment, and time of year.

Other Production Practices

Monoculture, the planting of the same crop in the same area year after year, can increase disease potential because residual material, spores, and insect eggs build up in the production area. Certain species of crops, such as tomato and potato, must be rotated as a unit. Both are solonaceous crops and are susceptible to many of the same pathogens. Windbreaks, such as sudan grass, can cut down on soil and dust scarring of plants and thus reduce entry wounds for pathogens. Tillage should be timed, depending on the crop, to aid in aeration and weed control without damaging roots or running over vines. Care should be taken when planting perennial crops to consider machinery size to avoid limb breakage, and to facilitate harvesting.

Use of bees to improve pollination and fruit set is needed in some crops. Blueberries, apples, stone fruits, and cucurbits benefit from hive placement in the field. Bumblebees can be purchased and used for pollination in place of honeybees. Poorly pollinated squash and watermelon become gourd-shaped and have poor internal fruit texture. Lack of pollination reduces fruit set in blueberries and tree fruits.

Harvest and Handling Practices

Harvesting

Harvesting fruits and vegetables directly affects appearance and shelf life. It's important to realize that fruits and vegetables are living organisms that respond to water, oxygen and carbon dioxide. Harvesting in the heat of the day causes wilt, shrivel, softness, and high respiration rates and shortens shelf life considerably. Rough handling, such as wounding from fingernail punctures, or bruising from throwing produce, creates areas that darken, soften, and are vulnerable to pathogen attack. Some commodities, such as white and sweet potatoes, and watermelon require a 'curing' period after harvest to toughen or suberize the skin (or stem of watermelon), enhancing resistance to wounds and disease penetration.

Ripeness stage at harvest determines the storage life of the product. For instance, cantaloupes harvested when the stem does not completely separate from the fruit (3/4 slip) have a storage life three times longer than that of fully ripe, yellow cantaloupes (see table). If the product will be marketed locally, then fruits at a more ripe stage are usually desired. If fruit must be transported long distances, then less ripe, firmer fruit must be used.

Don't take poor quality produce out of the field. This includes grossly misshapen, sunburned, injured, or decayed fruit and vegetables. Besides wasting transportation and disposal resources, storage of poor quality produce provides a source of decay innoculum for healthy produce.

Temperature Management

Cooling. Reducing the amount of cooling needed helps save time and money. Harvesting early in the morning, before plants become warm and respire heavily, cuts the amount of cooling needed and often lengthens storage life. Simple things, like placing fruit in the shade or using a white tarp to reflect heat from filled bins (leave at least two feet between the top of the bin and the tarp to get ventilation), can cut the load temperature by 30%.

Most fruits and vegetables require some type of cooling. The most common type of cooling is room cooling, where produce is placed in an insulated cold room and the heat of the product dissipates slowly with air circulation. Forced air cooling is less passive, and packaged produce is arranged around a fan system that pushes cold air through and around the produce to remove heat. This technique is often used for commodities that can't tolerate extensive handling or wetting. Packages must have adequate vents to get adequate air circulation and cooling. Hydrocooling, the immersion of produce in large quantities of chilled water, can be used for crops with a rind, such as cantaloupes, or for vegetables such as cabbage. Top icing is an expensive procedure, reserved for highly respiring crops like sweet corn and broccoli, and involves direct contact of slush ice with the product. The table on the following pages presents appropriate cooling methods for fruits and vegetables grown in the Southern United States.

Storage temperature. Storage temperature after cooling depends on the specific crop (see table on following pages). Most fruits and vegetables need to be stored at the lowest possible temperature to reduce water loss and prevent pathogen growth.

Some crops require an initial cooling period to remove field heat, but then must be stored at a warmer temperature to avoid chilling injury. Chilling injury occurs when fresh produce is held at a low temperature for a relatively long period of time and metabolic processes become backed up. Symptoms include water-soaked lesions, pitting, and exuding spots. Crops associated with subtropical or tropical areas are frequently chilling sensitive, such as citrus, kiwi fruit, banana, pepper. The table on the following pages presents appropriate storage temperatures for fruits and vegetables grown in the Southern United States.

Temperate-climate crops can also be sensitive but sensitivity is usually temperature and cultivar-dependent. For instance, some peach cultivars will get chilling injury when stored at 35-41 oF but not at 32 oF.

Cooling Methods and Storage Temperatures for Fruits and Vegetables Chart

Market Selection

One of the decisions that should be made before planting is the type of market aimed for. Each fruit and vegetable variety has a recommended use. Supermarkets want produce with a long shelf life, tolerant of temperature abuse, and generally firm or crisp. Customers of farm stands and pick-your-own operations want produce that tends to be fragile and has a short shelf life, such as fully ripe, highly flavored fruit or young and tender produce. Often, the more flavorful tomato and strawberry varieties are too soft for supermarket use but are highly esteemed by farm stand or pick-your-own customers.

Demographics can be important in marketing. Young, small squash is highly preferred in east coast markets while midwest markets prefer slightly larger squash. The type of customer and income determines the type of produce that will sell at a farm stand or pick-your-own (U-pick) farm. Young families generally have limited time but will pay more for produce. Older people often have favorite varieties remembered from their youth and will spend more time in harvesting or selection of produce. Often, regional associations will dictate the customer base. For instance, raspberries sold in the south will be purchased by transplanted northerners while blackberries sold in the south will be purchased more readily by the southerners.

Regardless of the market, two rules of thumb always apply. Don't sell junk and don't cut prices to beat the competition.

Sources of Information

Web sites

Address for the author: United States Department of Agriculture, Agricultural Research Service, South Central Agricultural Research Laboratory, Lane, Oklahoma


Back to Table of Contents