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%.
Long term effects of nematodes on six South African sugarcane varieties.
Newly released sugarcane varieties are routinely screened on sandy soils to assess their tolerance to nematodes. A site at Compensation in KwaZulu-Natal, South Africa, was chosen to assess the tolerance of N12, N31, N39, N41, N42 and NCo376. Varieties were either treated with Temik® at 20 kg/ha or left untreated. Yield, nematodes and soil and leaf nutrient status were assessed annually for seven years. Nematicide treated plots showed, on average, a 10 ton increase in cane yield and 1.3 ton increase in ERC yield per hectare over the seven year period, with the highest increase of 22 tc/ha in the fourth ratoon. All varieties tested responded to nematicide application and were rated as intermediate due to the 9-18% increase in yield. While yield differences were demonstrated between control and nematicide treated plots, differences in nematode community and nutrient status were not as distinct. Differences in yield between varieties were also observed. A difference of 8 tc/ha between N31 (lowest yielding) and N39, N41 and N42 (highest yielding) was noted in untreated plots. In general, varieties grouped together in couples with respect to the nematodes and nutrients associated with them. These groupings were NCo376 and N41, N31 and N42 and N12 and N39. Over time, the yield declined and this decline was consistent with climatic potential. A positive change in nematode community was also noted over time as the numbers of the plant parasitic (harmful) nematode Pratylenchus decreased and the free-living (beneficial) nematodes increased. All leaf nutrients except for nitrogen and silicon were present in sufficient amounts and therefore not growth limiting. Nitrogen levels however, were below critical levels from the second ratoon, and this could have affected yields. The data from this trial clearly illustrate the impact of nematodes on yields and the benefits of applying a nematicide.
Five New Tylenchorhynchus Species From West Africa
Five new species of Tylenchorhynchus from West Africa are described, namely T. triglyphus n. sp., T. ebriensis n. sp., T. clavicaudatus n. sp., T. sculptus, n. sp., and T. rhopalocercus n. sp.