Exclusionary tactics can only go so far in their efficacy; no crop system can remain pest-free for long. Even in an enclosed greenhouse, microscopic fungal spores float on air currents and may enter through vents. Workers may inadvertently bring plant pathogens into the structure on their hands or the bottom of their shoes and insect pests on their clothes. The same methods of introduction work just as well in a field grown crop. Complicating matters is the weed seed present in the soil and established weeds already harboring viruses, insect pests, and other plant pathogens. Insect pests like whitefly and thrips have been biding their time all summer long on weeds, ornamentals, and other alternate hosts. The same is true of plant pathogens too. Many common weeds, such as American black nightshade (Solanum americanum P. Mill) for example, are also hosts of plant viruses capable of infecting vegetable crops and causing reduced quality. Cucumber mosaic virus, Pepper mottle virus, Tobacco etch virus, and Tobacco mosaic virus are among the many viruses nightshade may harbor and under the right conditions be vectored into certain vegetable crops like tomato, pepper, and eggplant. Information regarding how to manage night shade may be found in the UF/IFAS EDIS Publication #HS1176, American Black Nightshade Biology and Control in Fruiting Vegetables, Cucurbits, and Small Fruits by Andrew W. MacRae.
Alternate hosts may be managed if detected but many viruses have unidentified or asymptomatic weed hosts. It is truly a race against time to get the crop up and established before pests and pathogens do. Anytime new seed or transplants are brought into a system, they serve as new sources of food for the local pest populations. Insect pests, plant pathogens, and wildlife respond to the opportunity of new sources of nourishment and move to take advantage of these grower inputs. Consequently, the second principle component of most IPM plans includes cultural control practices that can manipulate environmental conditions so crop needs are satisfied and pests are less attracted to the crop.
Some pests are attracted to stressed plants which are also often more prone to disease than their healthy counterparts. Growers often alter the soil pH by incorporating amendments prior to planting and utilize nutrient analyses to determine fertilizer application rates and timing so crop stress is minimized. Just as important, is avoiding crop growth which if too rapid, results in spindly plants with long internodes and excessively soft foliage which is attractive to a wide array of pests including aphids and others with piercing-sucking mouthparts.
Since good growing conditions are so critical, soil and bed uniformity become essential for irrigation and fertilizer placement, rate and timing so all plants in the field receive equal inputs and crop uniformity is maintained. Humidity which often plays a role in disease development can be reduced by adjusting crop density. Inside protected structures like greenhouses, vents may be opened to reduce humidity. These protected structures may also utilize cooling fans to reduce air temperature and crop stress. Another way growers may manipulate the farmscape environment is using reflective mulch which makes it difficult for insect pests to locate the crop. This is particularly useful at the beginning of the season as seedlings cast little shade and are lost in the glare of bright sunlight flying insects see. As the plants grow and shade more and more of the reflective plastic, the efficacy of this tactic lessens. In some cases, reflective mulch is impractical and other cultural practices need consideration. Such is the case in south Florida with sugarcane and vegetables where crop rotation is utilized to allow pest pressure to subside.