Introductie en vestiging van planteparasitaire aaltjes in nieuwe polders, in het bijzonder van Trichodorus teres

The soil of the newly reclaimed polders of the former Zuiderzee was initially found to be free from plant parasitic nematodes. This meant that the practice of crop rotation, as used with 'old' land, did not need to be applied for a number of years.After several years however, nematode populations became quite noticeable, sometimes to such a degree that the associated problems and restrictions have become important for agricultural management. The first infestations in the Northeastpolder, reclaimed in 1942, were already noticeable in 1949, in Eastern Flevoland, reclaimed in 1958, the first infestations were already noted in the neighbourhood of temporary housing at Lelystad and Roggebotsluis in 1959.Information has been collected from field research, field and laboratory trials in an attempt to discover the mode of entry of plant parasitic nematodes in the new polders. Special attention has been paid to the factors influencing their establishment and behaviour, the damage they can cause and the possibilities of control. Entrance and establishmentThe first infestations to be noted involved young trees, shrubs and perennials which had been brought in from 'old' land for afforestation and for planting along the roads, around the villages and farmhouses and on allotments (table 1, 2 and 3). Many species of plant parasitic nematodes were in large concentrations tranfered in this way. The road verges also appeared to be lines of contamination as infection can be caused by the soil fallen from transport carriers which often come from outside the polders. In addition, infection can accompany seeds and planting material (3.2.1).Moreover, research indicates a more general and scattered infestation, this infestation was already noticeable (fig. 2) before the soil was prepared for agricultural use. It is possible that introduction by birds or by wind may be responsible for light infections of the reed-vegetation whereby nematode populations can build up (3.2.4). Spreading by birds or wind could not, however, be demonstrated in our experiments.Just how far nematodes can settle and spread depends on a number of factors such as their way of reproduction (3.3.1), the presence of host plants (3.3.2) and on the soil type (3.3.3).It was curious that the first noticeable attacks of agricultural and horticultural crops were mainly caused by nematode species with an asexual reproduction. The establishment of bisexual species is possibly restricted because of the extremely low numbers of nematodes spread, when the question of socalled underpopulation can arise. Cyst-forming nematodes or nematodes present in soil clods or in the soil adhering to roots can be an ideal means of causing a local infestation.The influence of host crops, mainly expressed by the choice and sequence of the crops grown on farms, was clearly shown by a crop rotation trial (table 13). Quite independant of crop rotation, polyphagous nematode species, such as Trichodorus teres, demonstrated a rapid increase in population and a general distribution. On the other hand cyst-forming nematodes ( Heterodera spp. ) with a limited number of host crops, showed only a spreading which corresponded with the type of crops grown. Other species show an intermediate position which, dependant on the range of host-crops (table 11 and 13) offers the possibility of preventing or restricting damage by the use of crop rotation as is the case with cyst-forming species.The soil-type has also been shown to be of major importance for the establishment of nematode-species. The lighter soils are generally more suitable for the build up of populations from most plant parasitic nematodes. However, stem eelworms ( Ditylenchus dipsaci ) establish themselves better in heavier soils with more than 25% lutum (3.3.5.4.). It appears that there is an increase in the number of nematode species found in the soil the longer that it is in use for agriculture, whereby the initial favourable differences between reclaimed and 'old' land decrease. There is sometimes talk, from a nematological point of view, of unexpected rapid aging of soil which looses its favourable starting point.Damage to several crops grown in the lighter soils of the Wieringermeer and the Northeastpolder, was associated with populations of two Hemicycliophora species ( H. conida and H. thienemanni ) and of Trichodorus teres. Although these nematodes were previously unknown as plant parasites they have been studied more precisely as they have caused considerable damage in this area. Outside these polders only incidental damage is to be found in marine sandy soils, e.g. in the Braakmanpolder. These nematodes were also found to be present in similar soils on the isles of Texel and Terschelling and in the polder Makkummerwaard with both natural vegetation and grassland alike.Occurrence and behaviour of Hemicycliophora spp. and Trichodorus teresSerious damage caused by the two Hemicycliophora-species was first observed in the cultivation of a variety of crops on sandy soils near Ens in the Northeastpolder, principally carrot, bulbous iris and gladiolus. Later, it was also found in sugarbeet on other farms in the polder (3.3.5.6.). As these nematodes were found to be polyphagous, crop rotation offers little hope in damage prevention. The usual green manuring crops cause a population increase at such a rate that the growth of following susceptible crops becomes very risky. The use of organic matter, especially farmyard manure, has an inhibiting influence on population increase.The effect of nematicide treatments of the soil was so favourable in this area that it is generally applied before the most susceptible crops are grown. When susceptible crops are grown on this type of sandy soil elsewhere in the polders, one should take into account the possible presence of these nematodes.There was a remarkable difference in the occurrence of both species present in soil samples taken down the profil. H. conida was mostly found in the topsoil whereas H. thienemanni dominated the deeper layers. The population sizes of both species were greatest in the layer just below the tilth (fig. 4). In a trial using vertically placed tubes, this phenomenon could be reproduced. Results indicated a relationship between this difference of presence in the soil and soil moisture (table 18).A serious attack by T. teres finds expression mainly in an early stage of plant development as seen by stagnation of the growth, aberrant development of the roots and, as with potatoes and anemones, aberrant development of the sprouts. At this stage a definite relationship with the nematode population in the soil can always be found. Affected crops were more sensitive to periods of drought, produced a lower yield and an inferior quality or unfavourable grading. There were, moreover, differences in ripening of the crop (4.8).There was a clear connection between the poor growth of different crops caused by T.teres and the type of soil. This was mainly the case in soils with a low amount of silt and organic matter. The lower the siltpercentage of the soil, the more serious and more frequent was the damage. This was also the case with less susceptible crops (4.4.1). These differences are associated with a greater mobility of the nematodes in the soil (4.4.2). Nearly all pores in these marine sandy soils, especially where attack of T.teres is most serious, appear to be weak or unaggregated having a relative high pore percentage between 30-90 pm. As well as moisture, the pore size appears to be an important factor in determining the mobility of nematodes in the Soil (WALLACE 1963). Together with favourable moisture content and temperature, it means that these soils are a suitable environment for the maximum activity of free-living nematodes of which mainly T.teres has become so prominent (4.4.3). In soils with a silt and clay content above 12 % these nematodes are not or rarely found. Such top soils, when mixed with sand, lead to an increase of nematode populations (table 31 and 32), whereas, with the addition of clay to light soils, hopeful results were obtained regarding the rate of attack (4.9.3).As T.teres is polyphagous, crop rotation offers little prospect of practical prevention of damage (4.5.2). A very good reduction of these nematode populations in these soils was obtained with relative low doses of nematicides which resulted in marked improvement in root development and growth of susceptible crops (4.9.2). Populations of T.teres were not found to be reduced by application of large amounts of farmyard manure (4.6.3). T.teres is sensitive to soil disturbance so that tillage prior to sowing can give good results. However, this treatment carries the risk of soil structure deterioration (4.7.2).It is known that T.teres also acts as a vector for tobacco rattle virus. A correlation was found between the degree of virus infection and the type of crop which may proceed gladiolus cultivation. A higher rate of infection was found especially after cauliflower and, to lesser extend, after French beans and African marigolds than with some other flowerbulbs and vegetable crops (table 35). There was no obvious relationship between these differences and the size of the nematode population.After the growth of a number of agricultural crops (spring wheat, potatoes, sugarbeet, winter oilseed) and following fallow, a general transmission of rattle virus could be demonstrated from soil samples. In the case of potatoes grown in the field, symptom expression only followed winter oilseed. By using tobacco as bait plant and test plant a general virus infection was obtained from soil samples after the growth of perennial ryegrass and onions. The rate of transmission after spring barley grown under the same conditions was remarkably low (4.5.4.).To prevent virus-transmission a larger quantity of a nematicide is necessary than is the case with the control of root damage (4.9.2.). The symptoms of infection with rattle virus can be masked when farmyard manure is used in gladiolus cultivation (4.6.3.). Sufficient attention should be paid to the risk of infection with rattle virus when important crops susceptible for the infection (potatoes, gladiolus, tulips) are to be grown in these soils.