1. Soils   2. Climate   3. Other   General Information 

Genetic Variability
The reason “store bought” tomatoes taste different from “homegrown” tomatoes and why sweet corn tastes sweeter than sweet corn from the “good old days” is largely due to genetics. Different varieties of field corn are successfully grown from the Equator to northern Europe due to the genetic variability within the crop. (Obvious examples of human genetic variability are varied skin, hair, and eye coloring.) Genes control protein production including enzymes, special proteins that regulate chemical reactions. Different genes result in different proteins that affect plant physiology. In this way, varieties of the same crop may differ in their environmental requirements. Crop breeders develop new varieties by mixing and selecting plant types with desired traits.

Traits from several varieties can be combined into a single plant asexually, by grafting. For example, European grapes that are used to produce wine are susceptible to a root disease, phylloxera. Native North American grapes are not particularly good wine producers, but are resistant to phylloxera. By grafting European shoots onto American rootstocks, good traits from both types are combined. In addition to providing resistance to pests (fungi, virus, insects, and nematodes), grafting is used to extend the cold tolerance of a variety into cooler climates.

Indigenous people throughout the world have slowly, but effectively, selected the majority of crops we grow today. New crops may require domestication to realize their potential (yield, quality, and geographical range). Basic breeding work on some potential alternative crops, adapted to Illinois conditions, is currently being conducted by the Alternative Crops Research Program at Western Illinois University.

Growth Habit
A crop’s life cycle may be completed in less than one year (e.g., corn) or many years (e.g., alfalfa). The difference in life cycles is described by a crop’s growth habit. Annual crops (e.g., corn) complete their life cycle in less than one year. Biennial crops complete this process over two years. Biennial plants normally develop vegetative growth the first year and flower the second year. Perennial crops (e.g., apple) grow several years and flower and produce fruit each year. The amount and type of tissue that remains over time varies among crops. Some perennial crops grow new stems from roots each year while others, such as trees and shrubs, have persistent woody branches and limbs.

There are exceptions to these three main growth habit categories. Climate and other conditions may cause plants to have several growth habits. Pepper plants, grown, as annuals in Illinois gardens, are perennial plants in warmer climates. Functional annuals are biennial or perennial plants, such as carrots or tomato, grown as annual crops. Winter annuals (e.g., winter grains) are annual crops that are planted in the fall, overwinter in a dormant state, and finish developing in the spring.

Leaves are similar to solar panels in that they collect sunlight to store energy. Lignin connects cellulose to build the branches that anchor and spatially distribute leaves. Joining sugar molecules by removing a water molecule forms lignin. It is important to note that this reaction is not easily reversed, making lignin relatively inert. Therefore, in addition to gluing cellulose to form fibers and wood, lignin is a good initial barrier to keep pests out of a plant. Because the reaction is essentially irreversible, lignin is not available for plant energy. The amount of woody tissue must be balanced to plant needs. Excessive allocation of resources from storage into lignin production could jeopardize its survival.

Herbaceous plants, plants that make new stems each year, minimize lignin production to meet only annual needs. (Not all herbaceous plants are annuals. Many biennials and some perennials are herbaceous, surviving the winter as roots, rhizomes, bulbs, etc.). Lignin glues cellulose together to form fibrous tissues, but much of the shape of these plants is the result of water pressure. Woody plants (i.e., trees, shrubs, and woody vines) are more long-term investors in lignin. They convert more sugar into a durable structure framework that is used for several years.

It is important to know a crop’s wood content. Special equipment may be required to produce woody crops. A woody crop would be an undesirable partner in a rotation scheme because it is durable and its residue resists decomposition.

A weed is any plant growing where you do not want it to grow. This is subjective. Vigor and tolerance to adverse conditions are considered positive traits in crops but negative in weeds. ‘Crops’ such as Johnson grass and kudzu, have demonstrated that producers should be aware of a crop’s potential invasiveness before planting. Plant characteristics, such as shattering, production of many small seeds, and persistent roots or rhizomes, suggest a tendency to become a plant pest. Extremely invasive plants are listed as noxious weeds by federal and state agencies. The U.S. Department of Agriculture’s APHIS Web site provides lists of
federal and state noxious weeds.

Other concerns related to plant invasiveness and new crops are 1) the availability of effective, legal herbicides and 2) introduction of crops that are near relatives of existing weeds. Growing a crop without the assistance of chemical weed control can be difficult. Both sensitivity and insensitivity to common herbicides raise potential problems. Developing and gaining approval of new herbicides may take years and be prohibitively expensive. Labeling an existing herbicide for use on a new crop is also expensive, and the cost may dissuade manufacturers from attempting to get approval.

In addition to problems with chemical weed control, growing crops closely related to weeds entails risk of gene exchange among species. The danger of such transfer recently has been highlighted by the concern that transgenic herbicide resistance, among other traits, could be transferred to weedy species. Nontransgenic traits could be transferred as well. This exchange not only could make a weed more competitive, it may lessen the value of the crop if pollination by a weedy relative decreases the quantity or quality of the harvest or jeopardizes seed supply.

Poisonous Crops
Crops, by definition, benefit people. But not all beneficial plants are edible. Folk medicine and medicinal crops may contain alkaloids or other substances that affect human metabolism. For example, castor bean, a source of industrial oil, is poisonous. Rhubarb leaves are also poisonous, and sorghum can be poisonous to foraging livestock after a frost.

The U.S. Food & Drug Administration Web site, Center for Food Safety & Applied Nutrition, maintains a Poisonous Plant Database that includes a list of poisonous vascular plants. The Government of Canada, Minister of Supply and Services’ Canadian Poisonous Plants Information System provides links to sites listing poisonous plants including the University of Illinois at Urbana-Champaign Veterinary Medicine Library’s Web site, Plants Toxic To Animals.

Disease organisms are endemic to all regions. Whether these pathogens result in a disease outbreak depends on the susceptibility of the crop and the environment. Knowledge of the disease potential allows growers to weigh potential damage and take preventive control measures.

Diseases are often confused with their causal organism. The pathogen is an important part of a disease but it is the combination of the pathogen, a genetically susceptible host and a favorable environment that results in disease. These three elements make up the disease triangle. Plant pathologists sometimes add a fourth element, sufficient time for disease development, making a disease pyramid. Disease can be managed by eliminating one of the elements. Removing or burying debris to reduce the pathogen can control disease. Seed treatment may prevent infection and planting resistant varieties or rotating crops can reduce infestation. Some pathogens live through noncrop seasons on alternative hosts and can be controlled by eliminating the alternative host.Scheduling irrigation or timing planting to avoid environments favorable to infestation of the host can manage the environment.

 Disease Table  (Click to Enlarge)
The majority of important crops diseases are fungal. Viral crop pathogens are often vectored (transmitted) by insects and can be controlled by controlling the insect population. Nematodes, small soilborne worms, also cause important plant diseases.

Plant diseases may be associated with certain weather conditions, such as high humidity or specific temperatures. High humidity may promote pathogen spore production and distribution. Many pathogens, unable to penetrate crop plants themselves, rely on wind, rain, and hail to create an entry for pathogens.

Some diseases, such as wheat rust, are very genotype specific, requiring a specific strain or race of pathogen and a susceptible crop variety to form a disease. A single pair of genes, one in the host and one in the pathogen, may determine resistance or susceptibility. Other diseases, such as charcoal rot, are not as host specific, affecting many varieties and different crops. Obligate parasites, pathogens that require a living host, are host specific. Saprophytic pathogens that survive on dead tissue are likely to infect a greater number of crops.

Characteristics of common diseases are shown in the table. The crop susceptibility to these diseases is listed, where available, on each crop information page.

Geographical Classifications
Several conventions are used to list the geographical area for which a crop is best suited. These include the crop’s hardiness zone, climate zone, latitude and/or longitude, and geographic distribution. USDA Winter Hardiness Zones define areas where extreme minimum temperatures are similar. The U.S. National Arboretum "Web version" of the
USDA Plant Hardiness Zone Map (USDA Miscellaneous Publication No. 1475) shows "average annual minimum temperatures," based on the lowest temperatures recorded each of the years from 1974 to 1986. Isotherms are drawn every 5°F (-15°C). Many commonly grown horticulture cultivars are associated with the coldest zone in which they are grown. These representative plants are used as indicators of cold hardiness, facilitating comparison among crops. The entire map including Canada and Mexico is shown below along with a map showing Illinois only. The Illinois map is similar to the annual minimum temperature map (p=0.25) for 1971-2000.

Winter minimum temperature zones in Illinois

Koppen’s Climate Zones are a system widely used to classify climates. Included in this Web site’s crop database is the climate zone in which each crop either originated or is known to grow. The major classifications are listed below.

A - Tropical Moist Climates: all months have average temperatures above 64°F (18°C)
      Ar - Tropical wet climate
  Aw - Tropical wet and dry climate
B - Dry Climates: deficient precipitation during most of the year
  Bs - Steppe and semiarid climate
  Bw - Dessert and arid climate
C - Moist Mid-latitude Climates with Mild Winters
  Cf - Subtropical humid
  Cs - Subtropical dry summer
  Cw - Subtropical dry winter
D - Moist Mid-Latitude Climates with Cold Winters
  Do - Temperate oceanic
  Df - Temperate wet winters
  Dc - Temperate continental
  Dw - Temperate dry winters
E - Boreal Climate
F - Polar Climates: with extremely cold winters and summers

Okanagan University College, Texas A&M University-Kingsville, and Troy State University Montgomery maintain Web sites that provide additional details.

Latitude and/or longitude information is useful to describe a crop’s range or limits. While not providing specific crop requirement information, it does provide some insight, especially to those with general knowledge of an area’s climate. On the average, temperatures decrease at higher latitudes. In the eastern United States, precipitation decreases from the Atlantic Ocean and Gulf of Mexico to the Rocky Mountains. Precipitation in Illinois decreases with latitude.

The map on the right includes lines of longitude and latitude for Illinois.

Geographical Distribution
World Economic Crops lists geographical areas and regions for many crops. Areas with climates and soils similar to a crop’s listed growing areas are more likely to meet a crop’s requirements than other areas.

Economic Importance
Crop economic importance (
World Economic Crops) was grouped into five broad use classifications to allow rapid electronic access to economically important crops. The five searchable uses are described below:

  • Food – Cereal, fruit, nut, vegetable, pulse, juice, sugar, starch, bee plants, oil, beverage base, and additives for flavor, color, thickening, emulsification and other food for human consumption.
  • Animal Food – Animal food or feed including forage, fodder, and bird feed.
  • Medicinal – Plants containing chemically or medically active substances such as medicines, pharmaceuticals, folk remedies, poisons, and pesticides.
  • Material – Fiber, timber, gums, resins, latex, oils, dyestuffs, tannin, lipids, and wax.
  • Fuel – Rapidly growing plants for energy production, petroleum substitutes, and industrial oil.

Specific economic importance information, including uses with negative economic impact such as invasiveness, remain in the database and are displayed on individual crop information pages.

Metric-English Unit Conversions
Temperature and rainfall are commonly reported in both Metric and English units. Conversion of temperature between metric (°C) and English (°F) units is by the following equations:

            °F  =    (EC – 32)/1.8
            °C =    °F * 1.8 + 32

Conversion of rainfall from metric (millimeters) to English (inches) is by the following equations:
            in.      =    mm/25.4
            mm    =    in. * 25.4

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