Growing resistant potato cultivars is of little help in avoiding damage by spraing, because of regionally differing strains of tobacco rattle virus and of special requirements of the processing industry.On soil, fumigated in 1971 during autumn with a solution of dichloropropene-dichloropropane (660 g/l) at 2501/ha, the vectors Trichodorus spp., were effectively controlled and the tuber disorder did not appear in the next year's potato crop. But potatoes from the treated plots had an unacceptable off-flavour. In the second crop after treatment, control was still good and there was no difference in flavour between potatoes from treated and untreated plots.Disease incidence was significantly lower after spring barley than after sugarbeet, maize, or potatoes.
AddressPlantenziektenkundige Dienst, Geertjesweg 15, Wageningen, the Netherlands.
Book reviewP. Grijpma: Contributions to an integrated control programme of Hypsipyla grandella (Zeller) in Costa Rica. Diss. Agricultural University, Wageningen, 1974, 147 pp. This thesis is a collection of papers published from 1970 to 1974 in the Journal 'Turrialba'. The shoot borer Hypsipyla grandella is the main pest in yotmg plantations of mahogany (Swietenia spp.) and Spanish cedar (Cedrela spp.) in Latin America. The study focuses on the natural resistance to borer attack in mahogany and cedar. Two exotic species, African mahogany (Khaya ivorensis) and Australian cedar (Toona ciliata) were found to be immune to this pest. Two toxic components in the aqueous fraction of young leaves and shoots are held responsible for this resistance. Use of these exotic species could contribute to the solution of the shoot borer problem.
Soil receptivity as a quantifiable characteristic ranging from conduciveness to suppressiveness to soilborne pea pathogens Thielaviopsis basicola and Aphanomyces euteiches was determined by analysis of differences in disease response curves obtained by artificial introduction of inoculum into natural field soil samples. Several parameters, including maximum root rot severity, the area under the health index curve, scores on the first axis of a principal component analysis (PCA) on dose responses, and Weibull model fitting were used to describe the disease responses. In all cases, the Weibull model gave satisfactory fits. PCA yielded a first axis that comprised 86% of the variance found when using Weibull predicted responses for T. basicola and 74% of the variance found for A. euteiches. This PCA axis essentially represented the average increase in disease severity due to the addition of increasing doses of inoculum to the soil. The Weibull scale parameter B, which represents the amount of inoculum necessary to increase root rot severity by 63% with respect to the level caused by pathogens naturally present in the soil, is another means of quantifying the receptivity of soils to these plant pathogens. Weibull parameter B, maximum root rot severity, the areaunder the health index curve, and the scores on the first PCA axis were strongly correlated for each of the pathogens tested individually. To compare the extent and behavior of soil receptivity responses to different pathogens, Weibull parameters B and C (slope at dose B) were chosen because of their universal definition, in contrast to PCA scores. Comparison of the average levels of Weibull parameters B and C indicated significant differences between the pathogens. Yet, no significant similarity in the ranking of the soils was found for the three pathogens, demonstrating that individual soils may interact with different pathogens in totally different ways. In general, soils were suppressive to T. basicola but conducive to A. euteiches, whereas their response to Fusarium solani f. sp. pisi ranged from conducive to suppressive. Therefore, risk assessment of soils prior to planting may require different strategies for each pathogen. Bioassays with soil samples taken before the last pea crop in 1987 and 1991 revealed a significant increase in the natural inoculum potential of soils that mainly was accounted for by A. euteiches and Pythium spp. These results strongly indicate that A. euteiches must be considered one of the most threatening pathogens to pea crops in the Netherlands.
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