Biotic Stress and Yield Loss

one of the primary factors (rainfall being the other) which determines thelikelihood of plant water stress. The effects of water stress will be discussedin more detail later in the chapter.4.2.2 ABIOTIC AND BIOTIC STRESS-INDUCING FACTORS THATINFLUENCE CORN GROWTHThere are numerous abiotic factors that influence plant growth. Each category holdsthe potential for a multitude of stress-producing scenarios that influence yield independentlyor interactively. These factors are discussed because of their importancefor both singular and interactive influences on the primary determinants of cornyield—hybrid, planting date, and soil type. In addition, many of the plant stressesassociated with these factors are markedly similar to stresses caused by insect injuryto corn. A better understanding of the effects of insect injury on corn can be attainedthrough a basic understanding of the stresses caused by these abiotic and bioticfactors.4.2.2.1 Weather and Other Edaphic FactorsProductivity of a corn crop is greatly influenced by the seasonal growing environment.1 Weather is the most uncertain factor in farming. By itself, weather can makethe difference between failure and a bumper crop. 9 In grass crops such as corn, environmentalstresses during the time of rapid vegetative growth to achieve maximumphotosynthetic rates may delay leaf expansion, reduce photosynthesis, reduce wateravailability through surface evaporation rather than transpiration, and influenceflower initiation—all factors that can reduce corn yields. 1Adverse effects of weather come in a multitude of forms. Lack of water, toomuch water, temperatures too cool, temperatures too warm, impacts on nutrientuptake and utilization, and physical damage from strong winds, heavy rains, or hail,alone or in combination, can negatively (or positively) affect crop yields.4.2.2.1.1 Moisture stressStress may result from too much or too little water being available for the corn plant.However, a shortage of plant water is by far the most frequently occurring and detrimentalstressor. When the probabilities for water-stressed conditions are high, yieldpredictions (per the CERES–Maize corn growth model) decrease sharply. 7 Dry matteraccumulation is somehow closely related to the amount of water transpired by theplant, with less dry matter assimilation (including grain-fill) observed when transpirationis reduced. 1 It is known that dry weather conditions result in water stress,restrict root growth, and reduce or prevent adequate nutrient uptake. Corn leaf phosphorusand potassium levels are often reduced, even with high fertility programsand/or fertilizer additions—likely contributing to lower yields in dry years. 9Plants under water stress may be less able to compensate for pest injury thanplants that are fully hydrated. Additionally, plants suffering from water stress maybecome more (or less) attractive to arthropod pests. More specific impacts of waterstress on pest damage will be discussed on a pest-by-pest basis later in the chapter.