Cereal crops, resistance

In fields where some weeds were cleared using herbicides, other, more herbicide-resistant, species have appeared, such as common horsetail, coltsfoot, foxtail, wild oats, false wheat, etc. As a result of herbicide use, scratchweed, which cannot be destroyed by any herbicide, is making inroads into cereal crops, and chamomile has taken over rapeseed [6]. Using herbicides on rice fields caused the spread of wild, pesticide-resistant, low-yield forms of red-grain rice. 

Resistance of Cereal Crops to Aphids Role of Allelochemicals... 

Kotoula-Syka, E., C. Afentouli, and I. Georgoulas (2004). Herbicide-resistant weeds in cereal crops in Greece. 4th International Weed Science Congress, p. 50, S15MT08P00. 

It was later found that many carboxylic acids, provided that they were or/Ao-substituted, behaved similarly to the phenoxyacetic acids and many of these were marketed, offering small advantages in particular circumstances. A typical example is picloram (6.70) which was introduced in 1963 for controlling annual weeds and deep-rooted perennials. It is more potent than the phenoxyacetic acids and can destroy weeds in a cereal crop that are resistant to the phenoxyacetic acids (Kefford, 1966). 

Remarkable selectivity of action has now been achieved in some cases, such as the herbicides controlling the pest wild oats in cereal crops, and insecticides like Menazon attacking aphids but not bees and ladybirds. The increasing attention to selectiveness in activity also promises to reduce problems of resistance and cross-resistance which have been encountered with broad-spectrum pesticides like DDT and the organophosphates. 

A series of novel 4-phenoxyquinolines has been developed which show potent control of fungicide-sensitive and resistant strains of powdery mildew in grape and cereal crops. Protective, curative, and systemic activity has been observed for these materials, which also control other fungi. Strong activity in field studies has been noted and no crossresistance was encountered when fungicide-resistant strains of mildew were treated with these compounds, thereby suggesting a novel mode of action. The discovery, synthesis and structure-activity relationship of this novel class of fungicides are presented, as well as some data from field and cross-resistance studies. 

The two most used herbicides in this class are 2-methyl-4-chlorophen-oxyacetic acid (4.55) (MCPA or Methoxone )> and 2,4-dichlorophen-oxyacetic acid (2,4-D, or Chloroxone ). The use of 2,4,5-trichlorophen-oxyacetic acid has been suspended, because the usual process of manufacture contaminates it with a teratogenic dioxin (6.51). Tests carried out on 30 annual weeds that normally impoverish cereal crops showed that, by choosing the right compound and applying it at the right time, almost all the weeds can be killed. It is not known why cereals are relatively unaffected. In some experiments cereals were made to absorb as much of the phenoxyacetic acids as the weeds do (normally they absorb less), but the cereals remained unharmed (Wood, Wolfe and Irving, 1947). On the whole, dicotyledons are killed and monocotyledons survive, but there are striking exceptions. Thus onions (monocotyledons) are susceptible to the phenoxyacetic acids, whereas chickweed and cleavers (dicotyledons) are resistant because they have an enzyme that removes the side-chain. 

D (2,4-dichlorophenoxyacetic acid) A synthetic auxin used as a potent selective weedkiller. Monocotyledenous species with narrow erect leaves (e.g. cereals and grasses) are generally resistant to 2,4-D while dicotyledenous plants are often very susceptible. The compound is thus used for controlling weeds in cereal crops and lawns. See auxin. 

The most serious disease problems encountered in organic crops are blight in potatoes and seedbome diseases in cereals. Plant breeders are improving blight resistance in potatoes. As organically grown cereal seed cannot be treated, it is very important that it is tested for seed-borne diseases such as fusarium and smut. Foliar diseases in organic... 

Most of the ALS and ACCase herbicides have been introduced and used commercially only within the past 10-15 years and are often used repeatedly on the same land area. For example, several ALS inhibitors are used on com, while others in this class are used on soybean. Even though the crops are rotated and different herbicides are used, the different herbicides have the same mode or site of action, which increases the selection pressure for resistant weed populations. Various ALS inhibitors are now being used in many crops, including com, sorghum, soybean, and cereal grain. 

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