Influence of fertilization on biotic stress: control of bio-trophic pathogens by plant nutrition
Dr. Yigal Elad and Dr. Uri Yermiyahu
Start: Apr 2017 – End: Apr 2020
Mineral nutrition plays an important role in plants, as it is essential for their growth, development and reproduction. Balanced plant nutrition may influence yield quality and quantity in cultivated plants, as well as their ability to withstand biotic and abiotic stresses. Nutritional elements can affect plant susceptibility to plant pathogens (Engelhard 1989). The effect of plant nutrition on resistance to biotic stress is characteristic of the element, the plant species, and the pathogen. However, the recommendation to growers regarding plant nutrition in various plants is based on yield response of the crop and not on the effect of nutrition on plant diseases.
High nitrogen (N) rates enhance plant growth and foliage density. In general, increasing the N level is believed to result in increased susceptibility to B. cinerea. However, the results in this area are sometimes contradictory. Nitrogen supplied at 1.5, 3.8, and 6.0 g/m3 resulted in a quadratic increase in grey mould incidence on chrysanthemum flowers (Hobbs and Waters 1964). In contrast, Verhoeff (1968) found that the susceptibility of soil-grown tomatoes to grey mould increased with decreasing levels of N in the soil. Hoffland et al. (1999) found a linear correlation between the leaf C:N ratio and the susceptibility of tomato to B. cinerea. They attributed that correlation to levels of available soluble carbohydrates.
Among the most studied plant nutrition elements effect on plant diseases is calcium (Ca). Calcium is important for fruit and vine production (Ferguson 1984), affects various cell functions (Conway 1982) and increases resistance to disease (Volpin and Elad 1991; Conway et al. 1991). Calcium in plant tissues has been shown to reduce the severity of grey mould (Yermiyahu et al. 2006; Bar-Tal et al. 2001; Chardonnet and Doneche 1995; Volpin and Elad 1991). We tested in the past the effect of Ca fertigation in various crops and found it beneficial for the suppression of gray mold (Botrytis cinerea) (Elad and Volpin 1988, 1993; Volpin and Elad 1991).
In strawberry, increasing fruit Ca concentration reduced the incidence of Botrytis infection (Cheour et al. 1990; Karp and Starast 2002; Wojcik and Lewandowski 2003). Nevertheless, when conventional fungicides were replaced with CaCl2 to protect strawberries from B. cinerea, the CaCl2 treatments did not reduce the epidemic (Erincik et al. 1998). When Ca2+ was applied to table grapes in the vineyard, resistance to B. cinerea increased and this resistance was correlated with increased levels of cellulose and both oxalate- and alkali-soluble pectins (Miceli et al. 1999).
Recently, the effect of potassium (K) fertilization on gray mold was tested in sweet basil grown in pots, containers and soil. Increased K in the irrigation water and in the sweet basil tissue resulted in an exponential decrease in gray mold (Yermiyahu et al. 2015) and white mold (Sclerotinia sclerotiorum) (unpublished) severity. B. cinerea and S. sclerotiorum are necrotroph pathogens. Potassium supplied to plants by foliar application resulted in a significant decrease in gray mold and white mold in plants grown with a low rate of K fertigation. Lower K fertigation resulted in a significant increase in infection under semi-commercial conditions. Diseases severity in harvested shoots was negatively correlated with K concentration in the irrigation solution, revealing resistance to infection as a result of high K concentration in sweet basil tissue. Thus, proper K fertilization can replace some of the required chemical fungicide treatments and it may be integrated into gray and white molds management for improved disease suppression.
The use of adequate amounts of fertilizer, particularly K, has also been shown to reduce susceptibility to infection in field pea (Biddle 2001). Potassium is absorbed by plants in its cation form and it is the major cation in the cytoplasm. It is readily translocated in the phloem and xylem (Marschner 1986). A large number of enzymes are either completely dependent upon or stimulated by K (Suelter 1970). Interestingly, we found that fertigation with low levels of K is associated with higher levels of grey mould in sweet basil; whereas fertigation with comparably higher levels of K can suppress the disease under field conditions (Israeli et al. 2011; Yermiyahu et al. 2013). In grapevine, increased applications of K have been associated with less severe outbreaks of grey mould (Kiraly 1964) and, in pumpkin, increased applications of K have been associated with less severe white mould infections (Abia and Smith 1980).
With the current research we switch from the above necrotrophic pathogens (B. cinerea and S. sclerotiorum – pathogens that thrive on alive plant tissue and mainly on dead plant parts and residues) to biotrophs (pathogens that parasitize only live plant tissues) that are regarded as economically more important. We are currently studying the effect of nutrition on biotroph pathogens that represent major plant damaging agents, the powdery and downy mildews. We suggest continuing the research in sweet basil as a model plant and elaborating further the effect of nutrition on cucumber. Downy mildews are caused by oomycete microorganisms and powdery mildews are caused by fungi; many of them inflict significant damages on important crops. These diseases are promoted by humidity and moderate temperatures and the causal agents are biotrophs. Cucurbits downy mildew is caused by Pseudoperonospora cubensis. Powdery mildew of cucurbits is caused by Podosphaera xanthii.
The Cucurbitaceae family has great economic importance (worldwide annual cucumber production amounts to 23.2 million tons; Pitrat et al. 1999) and powdery mildew is one of the most important diseases of cucurbits worldwide (McGrath and Thomas 1996). This disease is mainly caused by P. xanthii. The typical sign of powdery mildew infection is a whitish powdery growth on leaf surfaces, petioles and stems. Infected leaves usually wilt and die, and infected plants senesce prematurely (McGrath and Thomas 1996). If uncontrolled, the disease reduces crop yield and quality. Therefore, protective fungicide treatments (mainly sulfur, azoles and strobilurins) are commonly applied to cucurbit crops. Frequent applications of azoles and strobilurins may increase the risk of the development of fungicide-resistant pathogen populations. Sulfur is toxic to predatory arthropods and, at high temperatures; it is also toxic to the crop.
As mentioned above, P. cubensis is the causal agent of downy mildew of plants in the Cucurbitaceae. Downy mildew of cucurbits can be found in temperate areas, tropical regions and some semi-arid regions. The disease affects cucurbit crops in the field and in greenhouses. Downy mildew is especially damaging in warm, humid climates. Downy mildew affects plants of all ages. The disease only infects foliage and reduces the photosynthetic activity causing yield reduction. Symptoms on cucumber are angular lesions that are limited by the leaf veins. During periods of leaf wetness the lesions can become conspicuously water-soaked; severe infection results in leaves that are completely dead and curled up (Palti 1971). P. cubensis is known to develop resistance to fungicides very rapidly. Reduced efficacy of mefenoxam, metalaxyl and the strobilurin fungicides has been reported (Gisi 2002). There is no significant information on the effect of plant nutrition on the downy and powdery mildews. We initiated research on sweet basil downy mildew (Peronospora belbahrii) because there is an outbreak of severe epidemics of this disease in recent years that threatened to wipe out the entire herbs growing industry in Israel. We intend to expend to the two biotrophic diseases mentioned above because of their importance.