**5.2 Efficacy of fungicides**

250 Fungicides for Plant and Animal Diseases

air floor to kill pathogens and spores. This alcohol derivative has often been used to reduce

The concentrations of sterilants and disinfectants widely used vary depending upon the nature of stock plants (soft vs. lignified plant surfaces) to be sterilized. A wide range of ethanol concentrations (20 – 100%) has been used, but high concentrations of ethanol (100% concentration) are rarely used, especially on soft stock plants because they can easily damage or injure the plant tissues. Generally, many epithytic contaminants (fungi) are eliminated, but not endophytes, and hence high concentrations of disinfectants are required to destroy these endophytic culture contaminants. Apart from the concentration of the disinfectants, exposure time must also be considered. Generally, the exposure time should be short (5 - 20 seconds), especially with the tender stock plants which are highly infested. Low concentrations of ethanol (≤50%) have often been used for tender and soft stock plants

For adequate plant sterilization (decontamination), stock plants are stirred in a beaker to ensure sufficient surface contact between stock plant and the fungicide. In this case, the exposure time, disinfectant concentration, and active ingredient (a.i.) are important factors determining the efficacy of the entire process of *in vitro* decontamination of stock plants. Rinsing the stock plants in the water assists in stopping the reaction between the chemicals (fungicide) and stock plants. The disinfection by fungicides and other surface sterilants in a sealed beaker (with aluminum foils) is followed by rinsing stock plants under running tap water for some time to remove the contaminants and also stop any reaction between the

Some fungicides commonly used to control plant culture contaminations during the *in vitro* culture include Benomyl, Captain and several others (Table 1). Utilization of these fungicides and other sterilants and disinfectants largely depends on their availability and the costs, especially in some Africa countries where these fungicides, sterilants and/or disinfectants are imported. Generally, they are not locally available in many African

The fungicides and disinfectants can be in the form of powder or liquid. Generally, the quantity of fungicides normally used per decontamination is very little. For instance, a low amount of Benomyl such as 0.14g per litre of water could be used to decontaminate some stock plants. Fungicides and disinfectants are harmful to human beings when swallowed or

In many plant culture laboratories, systematic fungicides (such as Benomyl) are preferred in case of endogenous fungi. Selection of a particular fungicide depends on the nature of fungal contaminants as some are hard to eliminate (e.g. endogenous fungi). The use of systemic fungicides (such as Benomyl) has often been recommended. In some cases,

inhaled. Therefore, proper handling of all fungicides must be a priority.

culture contaminants as it kills fungal pathogens and spores on the stock plants.

to avoid injuring the plant cells or tissues.

**4.1 Improving decontamination process** 

stock plants and the disinfectants.

**5.1 Selecting fungicides** 

**5. Selection and efficacy of fungicides** 

countries for use in plant culture laboratories.

Efficacy of fungicides depends on many factors including their active ingredient (a.i.), concentration (dosage), the type of stock plants (mature vs. new shoots), type of fungi (exogenous vs. endogenous), and the exposure time. Generally, rigorous decontamination is required for stock plants derived from mature ortet, especially when weaker disinfectants are used. This will also require longer exposure time. Generally, the use of concentrated (stronger) fungicides or disinfectants to eliminate contaminants from the stock plants requires a shorter exposure time.

Mercuric chloride as a culture disinfectant is stronger than sodium or calcium hypochlorite solutions for disinfecting stock plants and this could be the reason for its efficacy in decontaminating endogenous fungal contaminants. Danso et al. (2011) reported successful *in vitro* decontamination of sugarcane explants with mercuric chloride where NaOCl and Ca (OCl2)2 were less effective. Therefore, HgCl2 is ideal for the control of endophytic fungal culture contaminants.
