NEMATOLOGICAL ENVIRONMENT AND SELECTION PRESSURE ON SIX SASRI RESEARCH FARMS
Current estimates indicate that the loss in yield of sugarcane from root-feeding nematodes amounts to approximately R250 million per year. Tolerant varieties provide an inexpensive and long-lasting option to reduce crop loss from nematodes. Currently, genotypes are selected on the SASRI research farms without direct attention being given to their tolerance or susceptibility to nematodes. Not much is known about the nematode communities that exist on the research farms and whether these nematode communities exert sufficient selection pressure on the genotypes in the selection programme. The objectives of this work were (i) to characterise the plant breeding selection sites according to their nematode communities, and (ii) to measure the effect that nematodes have on the yield of varieties at plant breeding selection sites and thus identify the selection pressure for nematode tolerance. To achieve this, the 2nd stage single lines at six research farms were surveyed for nematode populations. Results showed that Empangeni, Gingindlovu and Kearsney had similar nematode profiles, whereas other research farms exhibited differences in nematode communities. For example, Bruyns Hill and Glenside had higher numbers of Helicotylenchus, whereas Pongola had higher numbers of Hemicycliophora and Rotylenchulus. The common variety at all sites, NCo376, exhibited a similar nematode community to other sugarcane clones at a particular site, but a substantially different community between sites. Yield data from single line trials showed that nematode communities influenced selection at four of the six sites. Two variety x nematicide trials were planted to assess the loss due to nematodes. At Empangeni, the yield responses (which reflect crop loss) in the plant crop ranged from 10 to 27%. At Gingindlovu, the responses in the plant crop ranged from 44 to 52%.
A REVISED ESTIMATE OF CROP LOSS IN SUGARCANE CAUSED BY NEMATODES
Previous estimates of crop loss from nematodes in the South African sugar industry were derived from data from conventional nematicide trials. These estimates were regarded as provisional as there could have been bias in the choice of sites. To avoid bias, a total of 48 crop loss trials were conducted over a period of three seasonsin fields that were not known to have a problem from nematodes. An additional criterion for site selection was that the soil had a clay content of about 10%. The soil at most of the sites was of a Cartref or Kroonstad form. Each trial comprised replicated, paired plots treated with a split application of 11-15 kg aldicarb/ha and an untreated control. The results confirmed that, in loamy sand and sandy loam soils, unsuspected losses from nematodes are widespread. The magnitude of the responses to treatment varied from season to season, from about 11% in a very dry season to about 32% when rainfall was close to the long term mean. Average losses for the industry from nematode damage are now estimated at more than 700000 tons cane per annum. Introduction Previous estimates of crop loss in sugarcane caused by plant parasitic nematodes in the South African sugar industry indicated that as much as 900000 tons of cane are lost each year (Spaull et al., 1990). The crop loss data were derived from conventional nematicide trials and were possibly liable to bias resulting from the selection of the sites. It was argued that, although any bias was probably small, the crop loss estimates should be regarded as the probable maxima until data from randomly selected sites wereavailable. This paper reports crop loss data from sites which, although not selected at random, are considered to be reasonably representativeof the soil typesto whichthey belong. It is wellknown that nematodes are a seriousgrowthconstraint in the Recent sands of the sugarindustry (Donaldson, 1985), but their importance in the more widespread loamy sand and sandy loam soils derived from Table Mountain sandstone is uncertain. Therefore the current crop losswork was focused on these slightly better soils. It was shown that treatment with a high rate of aldicarb (Temik 150G)increasedyieldsby about 25% over that from treatment with the rates used commercially (Spaull et al., 1990). A high rate of application was therefore used in all the crop loss trials reported here, although at some sites lower rates were also used. It is known that in the absenceof pestsaldicarb may affect the growth of a number of plants (Barkerand Powell, 1988; Barker et al., 1988; Garabedian and van Gundy, 1982; Griffin, 1988; Womack and Schuster, 1986; Kluge, 1992). Whether the effect of the chemical is positive or negativemaydepend on a number of factors (Barker et al., 1988). Initial work conducted in 1979 (*Spaull, unpublisheddata) indicated that aldicarb did not enhance the growthof sugarcane but.rather, that it was slightly phytotoxic when applied at high rates. However, in view of the work of Barker et al., (1988) ad28 ditional observations weremade on the effect of aldicarb on sugarcane. Theseare reported here togetherwith the original unpublished data. Methods Site selection Over the three seasons from 1991 to 1994 a total of 48 crop loss trials were conducted in 24 fields. Most were located in the North Coast extension region of Kwazulu-Natal. Nine of the fields were used for all three seasons, six were used for two seasons and the remainderforone season. Fields were selected if they were rated by the groweras not having a problem from nematodes, if they had a clay content of about 10% or more and, with one exception (see below), were available as recently cut ratoons. Site selection also ensured that most of the sites wereon TMS-derived Cartref form sands, loamy sands and sandy loams (Table I). Most of the remaining sites were on hydromorphic soils of the Kroonstad form whereinfield drainagewasnecessary. Except for one plant crop (CompI in the 1991-92 season) all the cropswereratoons.Mostof the fields had beenplanted with varieties NC0376 or NI2 (Table 1); a few fields had been planted with NI4 or N16. Soil samples were collected from each site for textural analysis. All the sites wererainfed except the Nkwln site, which was irrigated. Rainfall data were obtained from nearby meteorological stations. Trial procedure Paired plots, each 4 rows X 10m long and replicated 4 to 16 times, were marked out in each field (Table I). Each pair of plots was widely spaced from neighbouring pairs to ensure a wide range in nematode population size. Each of the pairs comprised ope plot treated with aldicarb at 16,8 g/IO m row (equivalentto 112gTemik 150G/1O m row)and one untreated plot. Row space varied from 1,1 to 1,5m so the rate of chemical applied ranged from 11,2 to 15,3 kg aldicarb/ha,withan average of13 kg/ha(= 75-102 kgTemik/ ha). Half of the chemical wasapplied by hand over the row, 2 to 13 weeks after harvest of the previous crop and the remainder 10 weeks later. In 36 of the 48 trials the previous crop was harvested in winter and the following ratoon crop treated beforeor during spring. Where the previous crop washarvested in spring the following crop was treated within six weeks. It had been shown in an earlier study that such delays between harvest and treatment of the following crop did not affect the efficacy of treatment witha nematicide(Spaull and Donaldson, 1983). The singleplant crop wastreated withinone weekofplanting and again 10 weeks later. At a number of the sites additional plots wereestablished adjacent to paired plots and treated with 4,2g aldicarb/l 0 m row. The row space in these sites ranged from 1,1 to 1,4 m, so the rate ranged from 3,0-3,8 kg aldicarb/ha with an average of about 3,4 kg/ha. * vw Spaull, Head Nematology Section, Pathology Dept, SASEX, Mt Edgecombe Proceedings of The South African Sugar Technologists' Association June 1995 Table 1 Summary of data for the trial sites Number of plots treated Soil Soil parent Clay Cane with £ 13 kg aldicarb per hectare group" material and % variety per season and Site soil form* 1991-92 1992-93 1993-94 TMS Comp3 TMS Cf 10,0 NCo376 8 CSEI TMS Cf 11,0 NCo376 8 7 II CSE2 TMS Cf 9,9 NI2 4 7 II CSE3 TMS Cf 7,9 NI2 8 Daml TMS Cf 10,0 Nco376 6 6 9 Elphl TMS Cf 9,6 NI2 8 8 10 Elph2 TMS Cf 11,3 NI2 6 8 8 Elph3 TMS Cf 8,0 NI2 7 Elph4 TMS Cf 8,2 NI2 5 8 10 Gill TMS Cf 8,5 NI4 8 Mlmth TMS Gs 18,8 NI2 6 Saxel TMS Cf 9,8 NCo376 6 8 9 Saxe2 TMS Cf 19,7 NI2 6 7 Saxe3 TMS Cf 11,4 NCo376 6 9 9 Zink TMS Cf 11,8 NCo376 6 6 Hydro Compl RS Kd 10,5 NI6 6 8 Comp2 All Kd 12,9 NI4 5 8 8 Comp5 All Kd 12,0 NCo376 6 6 Frazl RS Kd 14,1 NI6 12 Fraz2 RS Kd 11,8 NCo376 5 LM2 CII Kd 8,0 NCo376 16 14 LM5 MES Kd 18,8 NCo376 13 7 Mtunz All Ka 12,2 NI4 5 Nkwln CII nk 11,3 Nco376 7 n Soil group: TMS = soil derived from Table Mountain Sandstone. Hydro = hydromorphic soil. * Soil parent material and soil form: All = alluvium; CII = colluvium; MES = Middle Ecca sandstone; RS = Recent sand; TMS = Table Mountain sandstone; Cf = Cartref; Gs = Glenrosa; Kd = Kroonstad; Ka = Katspruit; nk = alluvial sands overlying Lower Ecca Shales. Soil samples were taken from the plots shortly before the first nematicide treatment, to determine the nature and abundance of the nematode populations. Additional samples were taken 10 and 20 weeks later. Each sample comprised 18 X 24 mm diameter cores per plot, to a depth of 220 mm. The nematodes were extracted from the soil by a combined decanting-sieving-Baermann tray method (Brown and Boag, 1988). The application of fertiliser and herbicide to the cane in the trial plots was done by the co-operator along with the rest of the field. Additional hand weeding was occasionally necessary. ,The plots at most of the sites were harvested after 11 to 14 months of growth. Exceptions were the 1992-93 CSE3 site and the 1992-93Comp2 and CSE2 sites, which were cut after 10 months, and the 1991-92 LM2 site and the 199293 Mlmth site, where the cane was 19 months old when harvested. Shortly before the cane was harvested, a count was made of the number of stalks in the 8 m net rows, i.e. the centre two rows excluding 1 m from each of the ends, ofthe treated and control plots. The cane was then sample-harvested as follows: two lots of 30 adjacent stalks were cut from predetermined points in each of the two net rows. The leaves and tops were removed, and the mass of the 120 stalks from each plot was recorded. A representative l2-stalk subsample 29 was collected for sucrose analysis. Mass of cane and sucrose/ ha were derived from these measurements. Influence ofaldicarb on sugarcane The effect of aldicarb on sugarcane in the absence of soil pests was measured in two pot trials, one on plant cane in 1979 and the other on a plant crop and the subsequent ratoon initiated in 1993. In the first trial, pre-germinated single-budded setts of variety NC0376 were planted in methyl bromide sterilised river sand in 260 X 300 X 100 mm deep seed trays in a glasshouse. Corn-cob-based granules of Temik l50G were applied as a single application in a shallow furrow in the sand over the setts, at rates equivalent to 0,21, 0,42 and 0,84 g aldicarb/m (or 1,75, 3,5 and 7,0 kg aldicarb/ha, assuming a 1,2 m row space). There were foursetts per tray and the treatments were replicated five times with two untreated controls. The plants were watered as required and fertilised with a hydroponic solution weekly. After 11 weeks the trial was terminated. The stalks and roots were weighed and the tips ofthe leaves indexed for the amount of scorching present on the leaf tip. In the second pot trial, pre-germinated single-budded setts of variety N12 were planted in methyl bromide sterilised soil from the LM2 site in 400 X 220 X 210 mm deep asbestos pots outdoors. After three weeks,gypsum-based granules of Temik l50G were applied as a single application on the surface of the soil, at a rate equivalent to 0,36 g aldicarb/ m (or 3 kg aldicarb/ha, assuming a 1,2
LAND DEGRADATION IN THE DEVELOPING WORLD: ISSUES AND POLICY OPTIONS FOR 2020
Land degradation takes a number of forms, including depletion of soil nutrients, salinization, agrochemical pollution, soil erosion, vegetative degradation as a result of overgrazing, and the cutting of forests for farmland. All of these types of degradation cause a decline in the productive capacity of the land, reducing potential yields. Farmers may need to use more inputs such as fertilizer or manure in order to maintain yields, or they may temporarily or permanently abandon some plots. Degradation may also induce farmers to convert land to lower-value uses. For example, farmers may plant cassava, which demands few nutrients, instead of maize, or convert cropland to grazing land.