The Role of Forecasting in Crop Protection Decision Making: an Economic Viewpoint1

In making crop protection decisions, farmers have to answer two questions: what type of control measure(s) should I adopt? How should I apply this (these) measure (s), when, how frequently, etc.? The answer to these questions will depend upon: a) the dimensions of pest attack and the damage it causes, b) farmers' goals, c) the range of protection measures available to them which they can use, and d) the information available, in the form of monitored and forecast information. It is within this context that the role of forecasting is considered. An analysis is made of the possible impact that forecast infomation can have on farmer decision-making, particularly with regard to the accuracy of the forecast and the time at which it is available. This leads on to the problem of assessing the value of a forecast at a regional level, against which the costs of providing such information can be set. Finally, situations in which forecast information is likely to be of greatest value (and of least value) are identified, and implications are drawn concerning the institutional and other changes that could be taken to allow better use to be made of forecast information. Lorsque les agriculteurs decident de proteger leurs cultures, ils doivent repondre aux deux questions suivantes: quel est le mode d'intervention a choisir et comment, quand et a frequence faut-il intervenir. La reponse dependra notamment: de I'etendue de I'infestation et de L'importance des deglts; des buts que s'est assignes L'arboriculteur; de la gamme de methodes dont il dispose; des previsions qui lui sont fournies. La prevision est condideree dans ce cadre, puisqu'elle est susceptible d'infuler sur les decisions de L'agriculteur dans la mesure ou elle est etablie avec precision et au bon moment. I1 est donc utile d'evaluer L'inter6t economique d'une prevision regionale en faisant intervenir le cofit de celle-ci. Enfin, il est fait etat de situations dans lesquelles la prevision est profitable ou non. Suivant les cas, certaines modifications structurelles peuvent s'imposer afin de mieux tirer benefice des previsions.     

Transmission of the agent causing a melon yellowing disease by the greenhouse whitefly Trialeurodes vaporariorum in southeast Spain

The agent causing a yellowing disease of melon (Cucumis melo), which results in severe losses in crops under plastic on the coastal plains of southeast Spain, was shown to be transmitted in a semipersistent manner by the greenhouse whitefly (Trialeurodes vaporariorum Westwood). The agent was transmitted by grafting, but not by mechanical inoculation or through seeds. The agent was acquired in the minimum period tested (2 h) and could infect plants in an infection feeding interval of 6 h. Capsella bursa-pastoris, Cucumis melo, C. sativus, Cucurbita moschata, Cichorium endivia, Lactuca sativa andTaraxacum officinale were found susceptible.Results suggest that the yellowing disease affecting melon crops in the southeast of Spain is due to a pathogen similar to beet pseudo yellows virus, but this has to be confirmed by serology.     

The incorporation of pathogen spores into rain-splash droplets: a modelling approach

Simple, theoretical, physical principles and existing experimental data were used to derive an analytical model to describe the incorporation of plant pathogen spores into splash droplets. Data were obtained from experiments on splash dispersal of spores of Pseudocercosporella herpotrichoides (cereal eyespot), Pyrenopeziza brassicae (oilseed rape light leaf spot) and Septoria nodorum (wheat glume blotch). In these experiments, incident drops of diameter 4–5 mm were allowed to fall onto spore suspensions 0.5 mm deep with 1.2 × 10⁵ to 6.5 × 10⁵ spores/mL. The analytical model was constructed as the product of three functions of droplet diameter which described, respectively, the frequency distribution of droplet diameters, the proportion of droplets carrying spores and the mean number of spores in spore-carrying droplets in each diameter category. The frequency distribution of droplet sizes was described by a log-normal distribution, the proportion of droplets carrying spores was described by an exponential function and the adimensional spore concentration in spore-carrying droplets was described by a power law. The cumulative proportions of spores in droplets in diameter categories of increasing diameter were calculated to compare observed and fitted data.     

Pathogenicity and Virulence of the Two Dutch VCGs of Verticillium dahliae to Woody Ornamentals

Two experiments were performed in two consecutive years to test whether isolates of different vegetative compatibility groups (VCGs) differ in their ability to cause disease in woody ornamentals, to study the host specificity of the isolates and to get an insight into disease development in woody hosts. A range of woody ornamental plant species, including Acer campestre, Acer platanoides, Acer pseudoplatanus, Catalpa bignonioides, Cotinus coggygria, Robinia pseudoacacia, Rosa canina, Syringa vulgaris and Tilia cordata, were root-dip inoculated with six isolates of Verticillium dahliae, belonging to the two VCGs that occur in the Netherlands (VCG NL-1 and VCG NL-2). Isolates belonging to each VCG caused severe symptoms of verticillium wilt in most plant species tested. Disease progress differed between plant species, but was generally the same for the two VCGs. No overall differences in virulence were observed between the two VCGs for external wilt symptoms, number of dead plants, or shoot length. No significant VCG × plant species interactions were present for these characteristics. However, isolates of VCG NL-1 caused more vascular discolouration than did isolates of VCG NL-2. Isolates within VCGs often differed considerably in their virulence to certain hosts, as shown by highly significant isolate × plant species interactions. Isolates were more virulent on their original host. These findings imply that VCG identification does not contribute to disease prediction for a range of woody hosts.     

Characterization ofSolanum tuberosum simple sequence repeats and application to potato culiwar identification

With the continued introduction of new potato cultivars, accurate identification is becoming difficult but is essential for maintaining cultivar integrity and Plant Breeders’ Rights. Hypervariable DNA sequences, referred to as simple sequence repeats (SSRs) or microsatellites, have been reported to be an excellent source of genetic markers. To determine the abundance, distribution, and composition of SSRs withinSolanium tuberosum, 252 sequences were searched for tetranucleotide and smaller SSRs with a minimum length of 20 nucleotides and a maximum discrepancy of two nucleotides. In total, 40 unique SSRs were observed in the 252S. tuberosum sequences examined and occurred at a frequency of one SSR every 8.1 kb. To assess the ability of site-specific amplified SSRs to identify potato cultivars, a simple (TCAC)_m and compound (TCAC)_m ? (CTT)_n SSR 5’ to the starch synthase gene and a compound (C)_p ? (CT)_q ? (AT)_r ? (G)_s SSR 5’ to the sequence encoding mature proteinase inhibitor I, were examined and shown to produce unique DNA profiles for 73 of 95 tetraploid cultivars. In total, 24 alleles were observed at these loci and the accurately sized amplified DNA products can be used to establish a database for cultivar identification. Site-specific amplified alleles were somatically stable and have been conserved in clonal variants of Russet Burbank independently maintained for almost seven decades, a characteristic essential for cultivar identification. As genetic markers, the abundant, informative, and easily examined site-specific amplified alleles of SSRs are ideal for quickly and accurately determining cultivar identity of S.tuberosum ssp.tuberosum.     

Direct penetration of Rhizopus stolonifer into stone fruits causing rhizopus rot

Rhizopus rot, caused by Rhizopus stolonifer, is a major postharvest disease of stone fruits. The disease is related to the occurrence of mechanical and physical damage; however, observations at a Brazilian wholesale market suggest that direct penetration can occur. Therefore, the penetration mechanisms of R. stolonifer in stone fruits were evaluated. To identify the production of enzymes that help with direct penetration by the pathogen, esterase activity, both in mycelial discs and in spore suspensions of the fungus in water and in modified Van Etten nutrient solution, was measured. Assays were also conducted to evaluate the growth of R. stolonifer on glucose or cutin as a sole carbon source. The pathogen grew on both media, and higher esterase activity was observed in the cutin medium. Wounded and unwounded peaches and nectarines were inoculated with R. stolonifer spore suspensions in water or in modified Van Etten nutrient solution. Wounded fruit inoculated with either of the R. stolonifer spore suspensions developed rhizopus rot, whereas unwounded fruit developed the rot only in the presence of spores in the modified Van Etten nutrient solution. Scanning electron and light microscopic examination showed the fungus can directly penetrate the nectarine cuticle. Diisopropyl fluorophosphate, a serine hydrolase inhibitor, prevented rot development in peaches. The results provide valuable evidence for the ability of R. stolonifer to directly penetrate unwounded stone fruits, probably due to the production of esterase enzymes.     

Identification and characterization of bacteria isolated from crown galls on stone fruits in Poland

Eighty stone fruit nurseries located in different regions of Poland were examined for the presence of crown gall affected plants. The disease was observed in 39 nurseries, and galls were sampled for bacterial isolation. Out of 1213 isolates, 409 were pre‐identified as Agrobacterium/Rhizobium spp. with 23S rDNA‐based multiplex PCR, and out of these, 315 were pathogenic when tested on sunflowers. Sequence analysis of three housekeeping genes (fusA, recA, rpoD) revealed that 366 strains belonged to Rhizobium rhizogenes, 23 to Agrobacterium tumefaciens species complex, and the rest of the strains were allocated to new phylogenetic lineages. Of these, the most numerous was the lineage allocated in the Pararhizobium genus. Positive results obtained from pathogenicity tests were generally in agreement with results obtained by PCR with primers complementary to T‐DNA except for two strains, which were positive for PCR but negative for the pathogenicity test. All detected Ti plasmids were nopaline‐type. Independent of their pathogenicity, 59% of tested strains were not sensitive to agrocin 84 in in vitro tests. Analysis of biochemical and physiological features distinguished 50 groups with different phenotypic profiles, but the tested traits were not consistent for strains classified to one taxon. This finding shows limited value of biochemical tests in identification procedures. The bacteria causing tumours were heterogeneous and strains classified to different taxa were found even in a single tumour.