Influence of variety on the within-plant distribution of cassava green spider mite (Acari: Tetranychidae), and leaf anatomical characteristics and chemical components in relation to varietal resistance

The within-plant distribution of the cassava green spider mite, Mononychellus tanajoa Bondar, and the anatomical characteristics and the chemical components relating to varietal resistance of cassava, were studied using 11 cassava genotypes with varying levels of resistance for two dry seasons and one wet season. The results show that M. tanajoa aggregates on the top leaves of cassava at low levels of resistance as compared with a more even within-plant distribution at higher levels of resistance. Thus, for accurate sampling of mite populations, it might be important to consider young and old leaves, not just the young leaves, in fields containing resistant and susceptible cassava genotypes. Correlations between the anatomical characteristics and mite population density and damage scores were not consistent across months, either within or over seasons. This suggests that leaf anatomical characteristics may not be important in the varietal resistance of cassava to M. tanajoa. During the dry season, mite population density was positively correlated with leaf nitrogen, potassium and phosphorus and negatively correlated with leaf calcium and fat. Only calcium was negatively associated with mite damage at the peak of the dry season (January 1993 and 1994). Breeding cassava genotypes with high levels of leaf calcium and fat, and low levels of leaf nitrogen, potassium and phosphorus, may improve the level of resistance to M. tanajoa.     

Tuta absoluta,a South American pest of tomato now in the EPPO region: biology,distribution and damage

Tuta absoluta (Meyrick) is an invasive pest of tomato native to South America, where it is responsible for extensive damage. It rapidly spread into several European countries, becoming a key pest. Knowledge of its main biological traits can be used to develop effective plant protection management strategies. It is a multivoltine species with a homodynamous behaviour, and the length of its life cycle depends on environmental conditions, particularly temperature. The larvae feed and develop inside tomato leaves, stems and fruits throughout the entire growing cycle. The adults have crepuscular habits, and in Mediterranean conditions they can be easily detected throughout the year. Damage is directly related to the reduction of plants' photosynthetic capacity and of production levels in both protected and open‐field tomato crops; indirect damage can be also caused by secondary infections, with pathogens developing on the infested plant and fruit tissues.     

A molecular marker linked to the Prv1 gene that confers resistance to Papaya ringspot virus-type W in melon

Papaya ringspot virus‐type W (PRSV‐W) is the most prevalent and important viral pathogen of cucurbits in Brazil. It can be effectively controlled by the incorporation of genetic resistance into susceptible melon cultivars. The present study identified amplified fragment length polymorphic (AFLP) markers linked to the PRSV‐W resistance Prv¹ allele. The susceptible yellow‐fleshed melon‐breeding line AF426^prv1 and its nearly isogenic‐resistant line AF426^Prv1, which carries the Prv¹ allele resident in the Indian cantaloupe U.S. Plant Introduction (PI) 180280, were screened for AFLP marker polymorphisms. Of 30 251 AFLP loci, only three were polymorphic between the nearly isogenic lines. Segregation analyses for these three polymorphic markers and the Prv¹ allele using a BC₁ population of 197 plants indicated close linkage (0.5% recombination frequency) between marker EK190 (HindIII‐CGA and MseI‐GTG; 190 bp) and Prv¹. Thus, EK190 might be a useful marker in breeding programmes aiming to develop melon cultivars resistant to PRSV‐W. The other two markers are closely linked to each other, but distantly linked to Prv¹.     

Persistence and colonization of gaps in sown swards of grass and clover under different sward managements

The effects of extensive sward management and patch size on the persistence and colonization of gaps in sown swards was examined by creating gaps of five different sizes (2·3, 7, 10, 14 and 19 cm in diameter) in four different sward treatments: a fertilized sward grazed to 4 cm, i.e. relatively intensive management, and three extensively managed unfertilized swards, which were not grazed or grazed to 4 cm or 8 cm. The swards were originally sown with ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.), but had developed differences in species composition as a result of the management treatments imposed 2 years before and during the experiment. Light quality measurements, i.e. red-far red (R/FR) ratio, were used to determine when the light environment in the gaps no longer differed from that in control, uncut patches and this was used as an estimate of gap persistence. Persistence of gaps depended on both sward management and gap size. Gaps disappeared most rapidly in the ungrazed sward and fertilized 4-cm sward, and most slowly in the unfertilized 8-cm sward. Small gaps persisted for up to 2 weeks in all but 8-cm swards, whereas larger gaps were estimated to persist for up to 20–25 weeks in unfertilized, grazed swards. There was no evidence that the number of grass or dicotyledonous species increased in the gaps compared with the control areas. There were significant positive linear relationships between the vegetation that developed in gaps and that in the control, uncut patches, reflecting the different species composition of the established sward of the grazed (grass-dominant) and ungrazed (Ranunculus repens-dominant) treatments. For total grass dry matter and tiller numbers, as well as L. perenne tiller numbers, there was a small, but significant, effect of both patch size and sward management on the slopes of the regressions between the controls and gaps. The results are discussed in relation to the potential for species composition of sown swards to change as a result of gap creation.     

The production of N-nitrosoiminodiacetate from nitrilotriacetate and nitrate by microorganisms growing in mixed culture

Mixed populations of microorganisms growing in a recycling soil percolator produced N-nitrosoiminodiacetate (NIDA) after 30 days growth using sodium nitrilotriacetate (NTA), a detergent additive, and NaNO₃ as C and N source respectively. Control experiments using autoclaved soil or substituting either acetate (C source) or NH⁺₄ (N source) did not show any N-nitroso derivative production. Five distinct strains of microorganisms were isolated which could grow on an NTA/NO^?₃ minimal medium but none produced NIDA in pure culture. Mixed cultures of two strains of Pseudomonas produced NIDA after 4 days percolation, using glass beads as a support. Cell-free extracts of the mixed cultures synthesized NIDA after 30 min incubation at 30°C with an activity of 27 μmoles NIDA produced/mg protein h^?1. Physical separation of the two organisms and the cell-free extracts by a dialysis membrane in a chamber determined that only one of the two organisms was responsible for the synthesis of NIDA, but the production was dependent upon the presence of the other organism. The possible relationship between the two organisms is discussed.