CHAPTER 1: INTRODUCTION
1.6 Control of seed-associated micro-organisms
1.6.1 Chemical control and its hazards
1.6.1.2 Categories of fungicides
There are two categories of fungicide, viz. contact and systemic fungicides. Contact fungicides, also known as protectant, non-systemic, or surface fungicides, are applied before host tissues are fungally invaded and act by killing or inhibiting the fungi (Yuste and Gostinear, 1999). The application of protective-contact fungicides alone following surface decontamination is ineffective when, as often occurs with recalcitrant seeds, the inoculum is located within the seed tissues (Berjak and Pammenter, 2004). They are, however, effective when the fungal inoculum is confined to the outer surfaces of seeds. In contrast, systemic fungicides must be taken up by the plants (which includes metabolically-active recalcitrant seeds) and translocated to the site of infection eventually killing fungi which have already penetrated the tissues. The recently developed systemic fungicides can act both as protectants and eradicants (Garcia et al., 2003), but one of the disadvantages of systemic fungicides is their site-specific action on pathogenic fungi, which can lead to the production of resistant strains, if they are not managed appropriately (Dias, 2012). Resistance to systemic fungicides is said to occur due to their specificity in acting against only one of the functions of the contaminants (e.g. benzimidazoles interfere with nuclear division and organophosphate fungicides inhibit chitin synthesis of the contaminants), rather than on a variety of functions. This apparently often leads to the development of new strains of contaminants that are resistant to one or another of the systemic fungicides (Agrios, 2005).
Use of systemic fungicides to extend storage longevity of recalcitrant seeds by minimising fungal proliferation is being tested in our laboratory. However, we are aware that their use may lead to various defects in the resultant plants, e.g. damage to foliar tissue (RPD No.1002, 2005), possible compromise of GA3 synthesis in wheat seedlings (Gao et al., 2000), interference with photosynthesis, as shown for Vitis vinifera (Saladin et al., 2003), and seed and seedling mortality, as reported for onion (Fullerton et al., 1995). Some examples of contact and systemic fungicides are listed in the table below:
8 Table 1.1 Different types of fungicides with examples (Agrios, 2005; Sambamurty, 2006; Singh and Singh, 2010) Fungicide type Group type Examples Contact Fungicide
Inorganic compounds
Bordeaux mixture (Biota Agro Solutions, India); Copper carbonate (CTM Supplies, England); bicarbonate sodium(BrunnerMont, United Kingdom), potassium(ArmandsProduct Company, South Africa); lithi carbonate (Private Account, South Africa); mono and dipotassium phosphates (Akash Purochem Private Lim India); elemental sulphur (Keg River Chemical Corporation, Canada), Lime sulphur (African Pegmatite, S Africa); mercuric andmercurouschloride(JMLoveridge, United Kingdom) sodiumhypochlorite (N Chlorchem, South Africa) and calcium hypochlorite (Aquachlor, South Africa). Organic compounds
Heterocyclic compounds e.g.: Captan (Universal Crop Protection, South Africa), iprodione (Bayer Crop Scienc Canada) and vinclozolin (BASF, United States). Dialkyldithiocarbamatese.g.:Thiram(BayerCrop Scienc United States), Ferbam(AmvacChemicalCorporation, California), Ziram(Swarup Chemicals,Indi Ethylenebisdithiocarbamatese.g.:Zineb (Universal Crop Protection, South Africa), Maneb (Environm ProtectionAgency, United States), metiram(EnvironmentalProtection Agency, United States), Mancoz (DuPontAgriculturalproducts, UnitedStates)Aromaticcompoundse.g.:PCNB(UniroyalChemical, U States), chlorothalonil (MakhteshimAgan, North America)and polychlorinatedbiphenyl (Chemcons Netherlands). Systemic Fungicide
Local systemicImazalil (Makhteshim Agan, North America), Triforine (American Cyanamid Company, United States) Partially or fully systemic Benzimidazole e.g.: Benlate (Villa Crop Protection, South Africa), Carbendazim (BASF, Germany), thiabenda (EnvironmentalProtectionAgency, United States)Other examplesof systemicfungicidegroups Triaz (AlliChem, United States), Strobilurins (Du Pont Agricultural products, United States), Oxanthiins (Crom Chemical, Switzerland), Acyclalanines (Environmental Protection Agency, United States).
Contact fungicides such as sodium hypochlorite (NaOCl) are used to treat post-harvest seeds or are applied as soil treatments for various crops, in view of the ability to kill, rather just minimise, the surface microflora (Ogawa and English, 1991). For most in vitro assays embryonic axes are recorded as being surface disinfected with 1% sodium hypochlorite solution containing 2 to 3 drops of a wetting agent Tween 20® before culturing (Dumet et al., 1997). The embryonic axes of T. dregeana in the present work were also disinfected in this way. Some other examples of other contact fungicides used on seeds are 3–30% hydrogen peroxide (Sutherland et al., 1987), calcium hypochlorite and alcohol (Pelczar et al., 1986), and, under laboratory conditions, 0.1% mercuric chloride is also used (Schmidt, 2000). It is also important to ascertain the ideal time of exposure to, and concentration of, the sterilants for seeds of individual species to avoid toxic effects (Schmidt, 2000). One of the advantages of using contact fungicides over systemic fungicides is that they are inexpensive and resistant strains of contaminants are less likely to develop (Dias, 2012). There are, nevertheless, advantages and disadvantages to the use of individual contact fungicides or groups of such preparations. Among the contact fungicides, organic fungicides are more effective and less toxic (Belpoggi et al., 2002; Hunsche et al., 2007). However, studies conducted in 27 countries have reported that dithiocarbamates, which are organic fungicides, were found to leave more residual contamination than any other type of fungicide (EFSA Scientific Report, 2009).
The fungicides that have attracted most attention for the post-harvest control of contaminants are Benlate, thiobendazole, Dichloron and imazalil; however, resistance of contaminants to thiobendazole (Tecto90®, Tecto 500 SC®) and imazalil (Fungiflor®, Magnate®, Deccozil®, Fungazil®, Imazagard®) has been reported (Holmes and Eckert, 1999; Conway et al., 1999).
Among the systemic fungicides, Benlate is the most used and effective chemical against a wide range of fungi causing plant diseases, and in crop protection (Garcia et al., 2003). It also acts as an effective contact fungicide for stored recalcitrant seeds such as those of T. dregeana (dusting on to the seed surfaces [Kioko, 2003]) as well as by coating seeds using a suspension; e.g. 0.3%
suspension of Benlate was highly effective in increasing the storage period of Hevea brasiliensis (Chin et al., 1988). In the present study various sterilants, viz. electrolyzed oxidizing water, Nipastat, Medi-chlor and Benlate were used to decontaminate the axes of T. dregeana. These treatments were also applied directly to the axenic cultures of fungal contaminants isolated from the axes.