The Connection Between Mildew and Calcium Nutrition in Agriculture

Introduction

Calcium is a major structural component of both cell walls and other plant membranes such as those surrounding organelles.  A shortage of calcium results in plant structures that are less able to physically resist infection by disease organisms, including downy and powdery mildew (Spectrum Analytic 2015). When properly functioning, cell walls also regulate the movement of sugars between plant structures such as leaves. When calcium is low, plant cells increase the transport of sugars into the intercellular spaces in the plant tissue, which increases the chances of infection and growth of disease pathogens (Spectrum Analytic 2015). Finally, as fungi and bacteria invade the plant tissue, they release pectolytic enzymes that dissolve parts of the plant tissue and allow for further infection. Interestingly, the deleterious activity of pectolytic enzymes is inhibited by the presence of calcium in plant tissues (Spectrum Analytic 2015).  In this article, I present evidence that links supplemental calcium nutrition with increased mildew infection resistance in susceptible crops.  First, we will review the two types of mildew most commonly found in agriculture.

Powdery vs. Downy Mildew (Western Farm Press – 2005; Penn State Extension Fact Sheets)

Powdery mildew: Powdery mildews are characterized by spots or patches of white to grayish growths resembling talcum powder. The fungus does not need the presence of water on the leaf surface for infection to occur. Powdery mildew prefers high relative humidity at night and low relative humidity during the day. Temperatures need to be moderate (70-80 ˚F). Each species of powdery mildew infects a specific crop.

• Spots are circular, not limited to leaf veins
• Fungus grows on leaf surface
• Spores produced in a single chain on a single stalk and grows on either the adaxial or abaxial leaf surface.

Downy mildew: Downy mildews are generally favored by cool temperatures (58-72° F), relative humidity above 85% at the leaf surface and moderate air movement. These conditions allow for spore dispersal and plant infection by the pathogen. There are several species of downy mildew and each species infects a specific crop family.

• Spots tend to be angular and limited by leaf veins
• Fungus grows inside the leaf and produces spores on the surface.
• Spores are produced on the end of branched stalks.
• The disease tends to be more visible on the underside of leaf surface (abaxial) as fungus sporulates through stomata in the leaf.

Supplemental calcium fertilizer is shown to have the following effects on mildew infection resistance:

• In a study of greenhouse grown cantaloupes, applications of foliar calcium + fungicide resulted in a 7.8% and 10.2% incident of downy and powdery mildew, respectively, 120 days after transplanting. This is significantly lower than the fungicide-only control incidence of 36.6% and 37.6% of downy and powdery mildew, respectively, 120 days after transplanting (El-Mougy et al., 2014).
• In a study of strawberries, low calcium content of leaves was correlated with a high incidence of strawberry powdery mildew infection and fungal growth rate. Leaves low in calcium also showed an increase in successful spore germination rate. Leaves with optimal calcium levels had lower powdery mildew levels and infection rates (Palmer 2004).
• In an experiment with greenhouse grown cucumbers, applications of foliar calcium + fungicide resulted in the highest reduction in foliar downy and powdery mildew disease incidence and severity. Furthermore, treatments with foliar calcium showed an increase the final yield of cucumber plants grown under greenhouse conditions relative to control plots (Abdel-Kader 2012).
• In an experiment with greenhouse tomatoes, a weekly spray of calcium nitrate or calcium chloride reduced mildew colonies by 90% to 95% compared to the water control.  A weekly spray of chelated calcium + calcium nitrate gave nearly 100% mildew control (Ehret et al. 2001).

- Karl Wyant, Division Agronomist, Western Business Unit

Recommendations

Supplemental calcium, in addition to a sound soil fertility and fungicide program, shows promising results at reducing both powdery and downy mildew infections in cropping systems. Additional calcium should be applied as a foliar spray or run through the fertigation system before climate conditions arrive that promote mildew spore production and population growth. Soil and plant tissues testing can help reveal possible deficiencies in calcium supply so that corrective action can take place.

References and Recommended Reading

http://westernfarmpress.com/dont-confuse-downy-powdery-mildew

http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/downy-mildew

http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/powdery-mildew

International Journal of Engineering and Innovative Technology (IJEIT)

Volume 3, Issue 9, March 2014, ISSN: 2277-3754, ISO 9001:2008 Certified

Protective Foliar Approaches against Downy and Powdery Mildews of Cantaloupe under Plastic Houses Conditions

Nehal S. El-Mougy, M.M. Abdel-Kader, S.M. Lashin, Australian Journal of Basic and Applied Sciences, 6(5): 249-259-, 2012, ISSN 1997-8178

Resistance Inducers Treatments against Downy and Powdery mildews of Cucumber under Commercial Plastic Houses Conditions

M.M. Abdel-Kader, N.S. El-Mougy and E.I. Embaby, Plant Pathology Dept., National Research Centre, Dokki 12622, Giza, Egypt

Foliar Calcium Sprays Reduce Powdery Mildew on Tomato

David L. Ehret*1, Carl Bogdanoff2, James G. Menzies3, and Raj Utkhede, HORTSCIENCE, VOL. 36(3), JUNE  2001

Strawberry powdery mildew: epidemiology and the effect of host nutrition on disease

Sarah Palmer, B. Ag. Sc (Hons), Thesis submitted in fulfillment of the requirements for the degree of Doctor of Philosophy (Agricultural Science) at The University of Adelaide Faculty of Sciences Discipline of Plant and Food Science School of Agriculture, Food and Wine Waite Campus, Adelaide, South Australia.

May 2007 Spectrum Analytic, Inc. “The Relationship Between Nutrients and Other Elements To Plant Diseases” Web. 20 September 2016