R O Clements and P J Murray
Institute of Grassland and Environmental Research, North Wyke Research Station, Okehampton, Devon EX20 2SB, UK.
Materials and methods
Discussion and conclusions
White clover (Trifolium repens) is an important sward component which in common with most other crop plants suffers from attack by a range of pest and disease organisms. The farming community remain largely unaware of the damage caused or of the organisms involved for a variety of reasons e.g. pests involved are often small or nocturnal, some are subterranean, diseases frequently form lesions on the underside of leaves or on other hidden parts of the plants and are easily overlooked.
Despite their low profile, many farmers and plant breeders increasingly recognise the importance of pests and diseases in limiting clover survival and competitive ability. However there was little firm evidence of which of the several pests and disease known to attack clover are the most prevalent. The present work surveyed damage by organisms that cause damage to clover foliage in England and Wales during a two year period.
Six widespread sites in England and Wales were selected for study in February 1990 and a further four were selected in August. All swards chosen had an easily visible clover content, had been established for at least five years, were grazed by sheep or cattle and otherwise managed appropriately for the various regions in which they were situated. Sites were located at or near Aberystwyth, Bronydd Mawr, Elham, Hurley, Myerscough, North Wyke, Penrith, Ruthin, Tadcaster and Worcester.
At monthly intervals, each field was walked and the presence or absence of patches caused by clover rot (Sclerotinia trifolii) noted. Twenty-four, 10 cm diameter turf cores were taken from clover-rich areas at each site from February through July 1990, but this was reduced to 15 cores per site from August 1990 through December 1991. The soil was washed from each core and the clover roots and debris discarded. For each core the youngest, fully expanded leaf from each growing point was examined and the damage caused by the major pests and diseases present recorded using keys modified from those of Lewis and Thomas (1991).
Some 53, 665 clover leaves were examined. Total damage i.e. % of leaf area removed or colonised by the various organisms, varied between dates and sites (P <0.001) but over all sites ranged from 7.6% (in July 1991) to 18.3% (in March 1990) and over all sites and all dates averaged some 11.9%. Damage by Sitona, molluscs, pigeons, Apion and leaf diseases averaged, over all sites and dates 7.5, 1.8, 0.4, 0.1, and 0.3% respectively. Damage by all organisms varied significantly between sites and between dates on all occasions (usually at the P <0.001 level). No infestation by Sclerotinia was observed in the field searches or more detailed assessments.
The average loss of photosynthetic area across all sites and dates of 11.9% is high, but probably underestimates the size of the problem. For example, it is recognised that the extent of damage caused by fungi extends well beyond the lesion and similarly insect and possibly mollusc saliva deposited around the edge of holes made by these pests causes disruption of plant tissue in that general area, and perhaps even systemically through the plant Further, molluscs were clearly present at most sites on most occasions since they damaged leaves and in all probability they also grazed and damaged growing points, but this was not recorded.
Damage by Sitona and Apion larvae would also probably be considerable and would be in addition to that recorded and caused by the adults. Also restricting assessment to the youngest, fully-expanded leaf, although being easily definable, minimises the period during which damage could have occurred.
Molluscs and more especially Sitona weevils emerged as the organisms causing most damage to leaves during this late winter through early summer period. No symptoms of Sclerotinia and few symptoms of nematode damage were recorded, but an absence of evidence of these organisms can not be taken as evidence of their absence. Sampling from clover-rich areas may have biased estimates of pest and disease incidence towards detection of more mobile species, causing instant damage, and away from organisms such as Sclerotinia and stem nematode whose infestations lead to very poor clover growth. Stem nematode occurred at 41% of sites in an earlier survey where poorly growing clover was sampled (Cook et al., 1992), compared with 33% in the present survey.
Pest and disease damage represents a continuous attrition of plant resources and was especially severe during the early spring period (exceeding 18% of leaf area removed in 1990) when clover plants are already under great environmental stress. It is pivotal that they survive and emerge from this period with the best possible chance of growing when conditions improve, but it appears to be the time when pest and disease damage is at its worst.
This work was done as part of that for a commission by the Ministry of Agriculture, Fisheries and Food.
LEWIS, G.C. and THOMAS, B.J (1991). Incidence and severity of pest disease damage to white clover foliage at 16 sites in England and Wales. Annals of Applied Biology. 188, 1-8.
COOK, R. MIZEN, K.A., PLOWRIGHT, R.A. and YORK, P.A. (1992) Observations on the incidence of plant parasitic nematodes in grassland in England and Wales. Grass and Forage Science, 47, 274-279.