Wide host range of plant species – Nematodes

Guidelines 

CropLife support the IRAC nematodes resistance statement which is as follows (and is available at: https://irac-online.org/irac-nematicide-moa-classification-now-available/): 

IRAC Nematode Working Group – Nematicide Mode of Action Classification 

Nematicide Resistance Risk statement 

There are no substantiated examples in the scientific literature from the last century documenting cases of significant tolerance shifts or suspected resistance leading to failure of commercial agricultural nematicides against plant parasitic nematodes (PPN) under natural field conditions. Instances of these phenomena occurring have only been reported for some products under controlled laboratory conditions(1). Product usage approaches and nematode ecology also reduce the potential that sustained selection pressure on PPN populations occurs under field conditions. Thus overall, it can be considered that the development of resistance in PPN species to nematicides under natural field conditions is currently unconfirmed, theoretically unlikely, and poses a low risk. 

The reasons underpinning this conclusion are explained below: 

Unlike other plant protection products (e.g. herbicides, fungicides and insecticides), several factors limit the potential for nematicides to create high and sustained selection pressure on plant parasitic nematode (PPN) populations under field conditions: 

These factors include the: 

• Relatively low frequency of nematicide use in a single cropping cycle, as a proportion of the duration of the crop and the number of PPN generations. Typically, one nematicide application is made per growing season, and occasionally more in long season or perennial crops. 
• Primary application methods used for nematicides in the field often target a small soil volume (e.g. crop root zone, crop beds or rows, or seed only), leaving untreated areas and host plants (weeds) that can act as refuge or source of recolonization for unexposed PPNs. 
• Various nematode species have life stages (dormant or living) in host plants (e.g. crop or weeds) that may remain in the field and not be exposed to or affected by nematicide treatments. It is noteworthy to mention that very few nematicides are effective systemically in the plant against nematodes. 
• Complexity of the soil environment and chemical interactions with nematicides frequently reduces product persistence, mobility and/or bioavailability, thus minimizing the likelihood of a chemical product to reach a high percentage of the plant parasitic nematode population present in the field, e.g. at different soil depths or distances from the point of application, or causing exposure to multiple generations. 
• Large diversity of naturally occurring organisms that may attack surviving life-stages of PPNs in soil, reducing the overall selection pressure from a single nematicide application. 

Plant parasitic nematodes occur in a variety of pressures (soil population density levels) under field conditions. In some countries, and in some species, local threshold levels may be available to assess the risk of economic crop loss. Nematode management programs should be used in cases where populations of PPNs are deemed high or very high, employing multiple tactics to provide effective control and population reduction. These programs may include cultural practices e.g. crop rotations or fallow periods, solarization, nematode resistant or tolerant varieties and the application of nematicides. In cropping systems which require multiple nematicide applications within one crop cycle or on the same field over several cycles, rotation to a nematicide with a different mode of action is recommended to reduce the risk of sustained selection pressure on PPN populations. 

Nematicidal products with fungicidal or insecticidal activity require additional resistance management considerations and labelling according to FRAC or IRAC guidelines. 

Reduced performance of chemical nematicides can be caused by the phenomenon of Enhanced Microbial Biodegradation (EMB)(2). This is well documented in the scientific literature and EMB should not be confused with resistance development in plant parasitic nematodes. EMB affects the level of product availability and duration of exposure of PPNs to the product, thus reducing the apparent efficacy of a nematicide application. Rotation of nematicides from different chemical classes, as well as employing other control methods such as resistant varieties and cultural methods (e.g. crop rotations) should be considered. 

1. Tolerance shifts or resistance development in PPNs under laboratory conditions:
   Although few cases have been reported, continuous exposure to sub-lethal levels of a single nematicide or mode of action may lead to the development of resistant populations under laboratory conditions. This however cannot be extrapolated to field conditions. 
2. Enhanced microbial biodegradation (EMB):
   Repeated or frequent use of the same chemical nematicide in the same field soil may lead to an apparent reduction in PPN control through enhanced microbial biodegradation (EMB) of the product. EMB is the result of adaptation and increase of microbial populations that break down a particular product, therefore changing the amount of product available and/or duration of exposure of PPN’s. The microbes responsible for EMB in soil may be different for different chemical classes or products, thus rotation of different nematicide types, or a reduction in the frequency of applications may decrease the likelihood of EMB occurrence. 

Nematicides registered in Australia