- Author: Jim Downer
Jim Downer is the Environmental Horticulture Advisor in Ventura County Cooperative Extension
As growers of thousands of ornamentals we understand that minerals absorbed mostly by roots as ions are essential for plant growth and development. Some required in parts per hundred are macro-nutrients while others only required in ppm or ppb are considered micronutrients. As long as enough of the 18 essential minerals are available, plants grow and reproduce in a healthful way. When not enough of one of the essential elements is supplied, a deficiency occurs and plants present symptoms. Mineral nutrient symptoms are considered abiotic disorders. There are however cases where excess or deficiency of elements can be predisposing to disease caused by pathogens. While examples of this are not abundant, some mineral elements do have a role in the development of disease caused by some pathogens.
Soil-borne pathogens are affected by minerals dissolved in soil solution. Minerals can act in specific ways (specific ion effects) or total ion effects (osmotic strength/concentration) having direct effect on pathogenic propagules or on the host itself. If we utilize the plant disease tetrahedron and think of all the implications ions could have in a biological disease relationship there are several possibilities:
-Specific ions harm the pathogen
-Specific ions harm the host
-Ionic strength changes the root environment making the host weak and susceptible
-Ions change the pH of the soil solution making it more/less fit for a pathogen or the host
-Ions change the soil physical environment making plants conducive to disease
While it is often espoused that the well “fed” or fertilized plant it is resistant to disease it is rarely borne out in any research. Keeping a good nutritional level in nursery stock will not necessarily protect plants from many of the virulent pathogens that are capable of causing disease. Nitrogen fertilization can produce succulent growth that will lead to exacerbation of such diseases as powdery mildew (Powel and Lindquist, 1997). Too many mineral nutrients may only result in luxury consumption by the fertilized plant or may cause other problems. It has long been known that seedling disease caused by Rhizoctonia solani increases with increased salinity in media (Baker, 1957) and later discovered by Jim Macdonald and others (1984), that salinity increases susceptibility of ornamental plants to Phytophthora.
Two basic plant resistance mechanisms that mineral nutrition can affect in plants are: 1.) formation of mechanical barriers (cell wall strengthening) and 2.) synthesis of defense compounds that protect against pathogens (Spann and Schumann, 2010). The role of specific elements and their compounds is much more complicated. Certainly deficiencies of molecules such as Calcium and Potassium can interrupt either of these defense mechanisms.
Root rot is a disease of thousands of ornamental plants and a serious problem in most nurseries. Root rots caused by Phytophthora spp. occur in a range of nutritional environments and pH’s. While some studies have implicated nitrogen compounds in the control of Phytophthora these probably involve the release of ammonia which is also toxic to plant roots (Zentmeyer, 1963). Lee and Zentmeyer (1982) later showed that both ammonium and nitrate reduced disease caused by P. cinnamomi , and that low levels of nitrate stimulated production of sporgangia. Most studies have found no relationship of nitrogen source to root rot disease development. Zentmeyer’s early work also suggested a role for calcium in disease reduction caused by Phytophthora root rots. Calcium increases disease resistance to root rot in Avocado (Duvenhage and Kotze, 1991). While it is understood that calcium has direct effects on plant membranes, root cell membrane leakage, cell wall thickness and many other host factors, Messenger (2000), later showed that the calcium ion also has direct effects on Phytophthora, reducing its sporangia size and zoospore mobility. Soils and media low in soluble calcium or where calcium is easily precipitated out of solution, or where pH is high and limestone minerals decrease the availability of calcium, are conducive to Phytophthora root rots.
Wilt diseases have also been studied in relation to disease occurrence. Keim and Humphry (1984) showed that nitrogen source reduced the incidence of wilt cause by Fusarium oxysporum f.sp. hebe in Veronica. In their system ammonium sulfate promoted disease and calcium nitrate prevented fusarium infections. In later work on the Fusarium oxysporum wilt disease of Canary Island date palm, Downer and others (2012) found no effect of fertilizer source on disease development (2013 ). Every disease system must be considered independently to determine if nutrient relationships are part of that system.
While it is easy to see a role for essential elements in plant defense, non-essential elements may also play a role in some systems. Silicon increases resistance of plants to powdery mildew (Kauss and others, 2003) and root roots (Cherife et al., 1994) and to stress in general (Ma, 2011). Silicon is implicated not only in strengthening cell walls but in defense protein production in plants (Faufeux et al., 2006). Not all plants are capable of utilizing silicon, so its role in plant defense is limited to those species capable of metabolizing it. Much more study is necessary to understand silicon’s role with ornamental plant –pathogen systems.
Nutrient exchange in container media is complicated-- it is mediated by the substrate, water chemistry, temperature and the applied ion sources as well as by plants growing in the media. Growers are well served to apply fertilizers that can supply a constant nutrient charge. Supply of extra soluble calcium, may be helpful in managing root rots. Avoidance of salt built up or high salinity situations that can occur when plant dry out will also help keep plants from
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Early onset root rot in color or bedding plants./span>