Herbicides and Plant Physiology. Andrew H. Cobb

to the crop, e.g. wild oats (Avena fatua) in cereal crops. Similar problems are encountered in the contamination of oilseed rape seed with seeds of weed species such as cleavers (Galium aparine). Where a proportion of the seed is saved for planting in subsequent seasons, this can cause a large increase in weed infestation. Contamination by poisonous seeds, such as darnel (Lolium temulentum) and corncockle (Agrostemma githago) in flour‐forming cereals is also unacceptable and once led to vastly increased costs of crop cleaning. Such cleaning, however, has meant that these weeds are now probably extinct in agroecosystems in the UK. A further example that still causes major problems is black nightshade fruit (Solanum nigrum) in pea crops (Hill, 1977). In this case, the poisonous weed berry is of similar size and shape to the crop and so must be eradicated. Although grazing animals avoid poisonous species in pasture (e.g. common ragwort, Senecio jacobea), they may be difficult to avoid in hay and silage, and some species, notably the wild onion (Allium vineale), can cause unacceptable flavours in milk and meat.

       1.4.4 Weeds as reservoirs for pests and diseases

Weeds, as examples of wild plants, form a part of a community of organisms in a given area. Consequently, they are food sources for some animals and are themselves susceptible to many pests and diseases. Because of their close association with crops, they may serve as important reservoirs or carriers of pests and pathogens, as exemplified in Table 1.6. Even where crop infestation does not occur, the presence of disease in weeds may cause problems, as is the case where grass weeds are infected with ergot (Claviceps purpurea), causing contamination of harvested grain with highly toxic ergot fragments.

Weeds may act as ‘green bridges’ for crop diseases, carrying the disease from one crop to another that is subsequently sown. Volunteer crops are particularly problematic in this case and can, in severe cases, negate the use of break crops as a cultural control measure for diseases. In addition, weeds can provide over‐wintering habitats for crop pests, resulting in quicker crop infestation in the spring. Ground cover provided by weeds can increase problems with slugs and with rodents, as the weeds provide greater cover and therefore reduced predation.

Table 1.6 Some examples of weeds as hosts for crop pests and diseases.

      Source: Hill, T.A. (1977) The Biology of Weeds. London: Edward Arnold.

Pathogen or pest		Weed		Crop
1.Fungi
Claviceps purpurea	(ergot)	Black‐grass	(Alopecurus myosuroides)	Wheat
Gaeumannomyces graminis	(take‐all)	Couch (Elytrigia repens)	Cereals
Plasmodiophora brassicae	(clubroot)	Many crucifers		Brassicas
2.Viruses
Tobacco ringspot		Dandelion	(Taraxacum officinale)	Tobacco
Cucumber mosaic		Chickweed	(Stellaria media)	Many crops
3. Nematodes
Ditylenchus dipsaci	(eelworm)	Chickweed	(Stellaria media)	Many crops
		Spurrey	(Spergula arvensis)
4. Insects
Aphis fabae	(black bean aphid)	Fat hen	(Chenopodium album)	Broad and field beans

      In 1994 and 1995 there were several severe outbreaks of the disease brown rot in potato in several European countries, especially in The Netherlands, which was possibly exported to other countries via infected seed potatoes. This extremely virulent pathogen (Pseudomonas solanacearum, syn. Burkholderia solanacearum, syn. Ralstonia solanacearum) causes a vascular ring rot in the developing tuber and causes a major loss of yield. Although often considered a soil‐borne organism, it was not found to persist for long periods in the soil following the harvest of infected crops. However, it was found to survive in the aquatic roots of infected woody nightshade (Solanum dulcamara) growing at the edge of irrigation channels. Thus, it may be the case that the pathogen overwinters in this wild host and is leaching into watercourses used to irrigate the crop, thus spreading the disease. This perennial plant is now being eradicated from potato‐growing areas. Several other species could also act as alternative hosts to the pathogen, including Solanum nigrum and Tusilago farfara, but further work is needed to confirm this.

1.5 Biology of weeds

      Knowledge of the biology of a weed species is essential to the design of management strategies for that weed. An understanding of the life cycle of a species can be exploited in order to identify vulnerable times when weed management and control might prove more successful.

       1.5.1 Growth strategies

According to Grime (1979), the amount of plant material in a given area is determined by two principal external factors, namely stress and disturbance. Stress phenomena include any factors that limit productivity, such as light, nutrient or water availability; and disturbance implies a reduction in biomass by factors such as cultivation, mowing or grazing. The intensity of both stress and disturbance can vary widely, with four possible combinations. However, only three growth strategies have evolved, as shown in Table 1.7. Although plants are unable to survive both highly stressed and disturbed environments, the other strategies have major significance to weed success.

      Ruderals are the most successful