DISEASE FOCUS: Effect of fertilizers on plant diseases
by Jim Downer
Ornamental plant growers understand that minerals are absorbed mostly by roots as ions, and are essential for plant growth and development. Some, required in parts per hundred, are macro-nutrients while others, only required in parts per million or parts per billion, 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 may 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 not common, some mineral elements do have a role in the development of disease caused by some pathogens.
Soil-borne pathogens are perhaps most affected by minerals dissolved in soil solution. Minerals can act in specific ways (specific ion effects) or as total ion effects (osmotic strength or 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 or favor the pathogen.
- Specific ions harm or support 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 or less fit for a pathogen or the host.
- Ions change the soil physical environment making it more or less fit for a pathogen or the host.
While it is often espoused that the well “fed” or fertilized plant is resistant to disease, it is rarely borne out in published 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 (Powell and Lindquist 1997). Excess mineral nutrients may result in luxury consumption by the fertilized plant or may lead to other problems. It is well known that seedling diseases caused by Rhizoctonia solani are more severe with increased salinity in media (Baker 1957), and it was later discovered by Jim MacDonald and others (1984) that salinity increases susceptibility of ornamental plants to Phytophthora.
Plant mineral nutrition has effects on two basic mechanisms of disease resistance: (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 complicated and unique to each disease/host system. 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 many nurseries. Root rots caused by Phytophthora spp. occur in a range of nutritional and pH environments. While some studies have implicated nitrogen compounds in the control of Phytophthora diseases, 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, but that low levels of nitrate stimulated production of sporangia. 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 cations increase 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. When soils and soiless media are low in soluble calcium, when calcium is easily precipitated out of solution, or when the pH is high and limestone minerals decrease the availability of calcium, conditions are conducive to Phytophthora root rots. Sodium in soils and soiless media can also affect Phytophthora.
Early onset root rot in bedding plants. Photo by Jim Downer.
Wilt diseases have also been studied in relation to disease occurrence. Keim and Humphrey (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), 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. Preventing salt build up or high salinity situations that can occur when media dries out will also help plants avoid infection by root rot organisms.
James Downer is Environmental Horticulture Farm Advisor, UC Cooperative Extension, Ventura County.
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