Sustainable agricultural practices promote low pesticide input farming and integrated pest management programs. It is generally agreed that the overuse and misuse of conventional pesticides has placed our foodchain and environment at risk and has contributed to a rise in pesticide resistance. Although few plant diseases are completely controlled by the application of fertilizers, Botrytis and many other diseases can be alleviated by the proper application and management of plant micronutrients. There is much scientific support that micronutrients, such as calcium, copper, boron,
manganese, zinc and iron, can reduce the severity of plant diseases by increasing disease tolerance and the resistance of plants to pathogens. Just as propernutrition strengthens human health, an integrated pest management program should incorporate proper nutrition as well as an appropriate use of non-traditional soil and plant amendments, such as seaweed and kelp, to strengthen defense mechanisms in plants.
The focus of a fertility program should be on providing appropriate nutrition and preventing deficiencies, rather than waiting for the deficiency to occur. The current
micronutrient critical values used in soil and tissue testing are outdated and inadequate for maximizing production. A potential exists for sampling errors due to in-field variation in nutrient distribution and the non-uniform distribution of plant roots. Micronutrient availability is dependent on weather, root activity, translocation, and microbial activity. Cold soils in the spring can result in low microbial and root activity, low micronutrient uptake and micronutrient cations moving slowly in soils. The micronutrients, iron, manganese, copper and zinc, demonstrate poor translocation in plants, causing the micronutrients to move only about one quarter inch in the plant. As a result, during periods of high plant growth (i.e. flowering), it is common for
plants to exhibit transitory micronutrient deficiencies.
There are two types of immunizing responses in a plant: Systemic Acquired Resistance (SAR), a direct response to infection, and Induced Systemic Resistance (ISR), a response to some exogenous chemical with no associated infection. When a plant is nutrient deficient, its defense responses can become impaired.
Micronutrients can control or reduce pathogen damage to the plant by direct toxicity to the pathogen or by promoting ISR. Applications of nutrients such as manganese, copper and boron can release, through cation exchange, calcium from ce ll walls. Once released, the calcium ions act together with salicylic acid to trigger a SAR response (Reuveni et al. 1997a, b; Reuveni and Reuveni 1998).
Studies have shown that nutrients can play a significant role in disease prevention. As stated above, calcium is important for the structure of plant membranes and cell walls (Marschner 1995). Low levels of calcium make the plant’s cel l wall susceptible to fungal attacks, while proper levels of calcium in plant tissue protects fruit from pathogens during storage. The application of calcium
to fruit prior to storage can be an effective measure for preventing fruit rot. As the ratio of potassium to calcium increases, so does the risk and severity of Botrytis infection (Krauss 1971). The incidence of Botrytis in table grapes is reduced when the K:Ca ration is 0.82:1. Calcium also strengthens the plant’s resistance to Pythium, Sclerotinia, Botrytis and Fusarium (Graham 1983). The timing of calcium and micronutrient applications is very important. Plant pathogens often infect the plant long before the actual symptoms of the disease are expressed. Control of the pathogens should start prior to infection to reduce the potential inoculums levels at later stages of fruit development. The use of sulfur and copper fungicides are good examples of this practice. Calcium
can inhibit pectinace activity of Botrytis cinerea (Volpin and Elad 1991). Botrytis infects grape vines through aborted flowers at bloom. Therefore, the application of calcium at bloom may reduce the severity of Botrytis bunch rot.
By: Dr. Larry Parker, Westbridge Agricultural Products