Introduction

The phenomenon of guttation is interesting, as it provides a wide range of possibilities for evaluating its economic significance. Most of the guttate consists of growth-promoting substances for various kinds of facultative parasites and saprophytes including bacteria, virus, fungus, and algae. The exudate's pH also seems to be moderate and favorable for the growth of microbes. As described in Chapter 5, the guttation fluid may contain a number of organic substances such as proteins, peptides, enzymes, amino acids, amides, sugars, vitamins, nucleotides, RNAs, lipids, alkaloids, glycosides, and reducing and oxidizing agents (Gay and Tuzun 2000; Goatley and Lewis 1966; Grunwald et al. 2003; Pilot et al. 2004; Singh and Singh 2013); inorganic constituents such as ions, minerals, and salts; and specific substances that promote growth, such as auxins, gibberellins, cytokinins, and abscisic acid (Aloni 2010; Aloni et al. 2005; Daviere and Achard 2013; Fletcher and Mader 2007; Muller and Leyser 2011; Thompson et al. 2007), which in turn result in the initiation of certain pathologic processes (Singh 2014b; Singh and Singh 2013). Johnson (1936) highlighted the relationship between root pressure, guttation, and plant diseases. He showed that because of guttation, the leaves get soaked in water for a longer period, which favors pathogenic infection in the plant parts. It is commonly observed that besides being a liquid vehicle for pathogenic microorganisms to invade the host cells in the plant, the guttate leads to increased relative humidity in the surrounding of the plant parts, which is similar to the effect caused by dewfall or other forms of precipitation (Hughes and Brimblecombe 1994). Guttation, therefore, should be considered an important issue while attempting to explain disease epidemiology of agricultural and horticultural crops in both irrigated and rain-fed regions (Brandl and Amundson 2008). While investigating guttation, it should be borne in mind that there is a relevant difference between dewfall and the guttation process. The former being intrinsic to the plant while the latter relates to the atmosphere; they usually occur at different periods. Understandably, repeated guttation occurs in the form of water films and droplets in a free state along the leaf margins as well as on the surface, which might provide favorable conditions for infection, which are described below in detail.