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April 2016 WINES&VINES 75 PRACTICAL WINERY & VINEYARD GRAPEGROWING S ustainable agricultural practices promote low-pesticide-input farming and integrated pest management programs. It is generally agreed that when overused and misused, conventional pesticides contribute 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 allevi- ated by the proper application and manage- ment of plant calcium and micronutrients. 28 There is much scientific data to support that micronutrients such as copper, boron, manga- nese, zinc and iron can reduce the severity of plant diseases by increasing disease tolerance and resistance of plants to pathogens. Just as proper nutrition strengthens human health, an integrated pest management pro- gram should incorporate proper nutrition and appropriate use of non-traditional soil and plant amendments, such as seaweed and kelp, to strengthen defense mechanisms in plants. 29,30 The focus of a fertility program should be on providing appropriate nutrition and pre- venting deficiencies, rather than waiting for the deficiency to occur. Better tools are needed for testing micronutrients, and the current micronutrient critical values used in soil and tissue testing need to be updated to meet cur- rent production levels. 1 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 the physical, chemical and biological condi- tions of soil, weather, root activity, transloca- tion and microbial activity. Cold soils in the spring can result in low microbial and root activity, low micronutrient uptake and micro- nutrient cations moving slowly in soils. The micronutrients, iron, manganese, copper and zinc demonstrate poor translocation in plants, causing the micronutrients to move only about 0.25 inches in the plant. As a result, during periods of high plant growth (such as flower- ing), it is common for plants to exhibit transi- tory micronutrient deficiencies. There are two types of immunizing re- sponses in a plant: systemic acquired resistance (SAR), a direct response to infection, and in- duced 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. 31 Applications of nutrients such as manganese, copper and boron can, through cation exchange, release calcium from cell walls. 2,3 Once released, the calcium ions act together with salicylic acid to trigger a SAR response. 2,3 Calcium 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. 4 Low levels of calcium make the plant's cell wall suscep- tible to fungal attacks, while proper levels of calcium in plant tissue protect fruit from patho- gens during storage. The application of cal- cium to fruit prior to storage can be an effective measure for preventing fruit rot. As the ratio of potassium to calcium in- creases, so does the risk and severity of Botrytis infection. 5,6a High-potassium and low-calcium conditions increase disease caused by Phytoph- thora parasitica. 6 Calcium and magnesium affect potassium availability; conversely, potas- sium can affect calcium availability. 7,8 High levels of potassium can limit calcium and magnesium absorption because they have lower transport rates through the plasma membrane. 32 Therefore, it may be important to apply calcium with late-season potassium sprays (to increase fruit sugar levels) to reduce late-season Botrytis severity. It is also impor- tant early in the growing season, when foliar fertilizers are often applied with fungicide; the demand for calcium in growing shoot tissues is high, and risk of diseases involving moisture is high. The incidence of Botrytis in table grapes is reduced when the potassium:calcium ratio is 0.82:1. Calcium also strengthens the plant's resistance to Botrytis, Pythium, Sclero- tinia and Fusarium. 2,5 When to apply calcium and micronutrients is very important. Plant pathogens often infect the plant long before the actual symptoms of the disease are expressed. Control of the patho- gens should start prior to infection to reduce the potential inoculum levels at later stages of fruit development. Application of lime sulfur early in the sea- son helps increase calcium levels in the vine and helps delay the onset of powdery mildew infestations. 16 It has been hypothesized that high concentrations of calcium oxalate in the stigmatic and stylar tissues of the grape flower prevent germ tube elongation by Botrytis cine- rea. 9,10,11,12 Calcium also has been shown to decrease the decay area (severity) caused by Botrytis cinerea in apples. 13 Induced systemic resistance pathways can be turned on by the presence of ergosterol (commonly produced by plant pathogens) and Micronutrients and Calcium Can Reduce Botrytis in Grapevines Uncovering the uses of manganese, zinc, boron, seaweed and humic acid extracts to fight disease By Dr. Larry Parker