The effect of environmental factors on the development of each stage ofPeronospora destructor (Berk.) Caspary on onions is reviewed. For sporulation to take place, a period of light must precede the period of darkness and high humidity in which spores are formed. Spores are discharged when the relative humidity (RH) is increasing or decreasing, and over a wide range of temperatures. Their discharge is triggered by exposure to red-infrared radiation and by vibration of the leaf. Dissemination of spores follows a daily periodic cycle and spores can be blown by wind over long distances. Duration of spore survival depends on temperature, RH and, especially, the absence of strong radiation. The rate of spore germination is highest at 10°C and declines with the rise in temperature. Germ tubes develop in liquid water, and a continuous period of wetness is required for infection to be completed. Systemic infection is common in cooler climates, where necks of onion bulbs are slow to dry. The principal sources of downy mildew infection by wind-borne spores are systemically infected propagation material, onion volunteer plants, and neighboring older crops. Development of mildew epidemics depends primarily on moisture, but the different stages in the infection cycle are favored by various kinds of moisture (water, high RH, or changes in RH levels). The temperature optimal for epidemics is 10-12°C, but temperatures up to approx. 22°C can support rapid disease development. Light is essential to build up photosynthates for subsequent sporulation. Shade plays a complex role, especially where dew supplies most of the moisture required. Where this is the case, under semi-arid and rainless conditions, favorable early morning temperatures may compensate the pathogen for the limited hours of high humidity, and thus promote severe mildew outbreaks. Reliable methods of predicting onion mildew are not currently available, but disease-free periods can be defined empirically by multi-annual observations. Control recommendations include securing propagation material free from systemic infection, destruction of volunteer plants and onion refuse, avoiding proximity of infected crops, choosing sites with relatively short dew periods(e.g. southern slopes), avoiding excessively dense stands, and irrigation preferably by methods other than overhead sprinkling. Onion mildew can be effectively controlled by fungicides; considerations for correct timing of their application are discussed.

Advisory work, by its very nature, is an intermediary between the re search worker and those who apply the results of his research. The challenge of advisory work is to devise means of and find pathwa

S OF PAPERS PRESENTED AT THE 10TH CONGRESS OF THE PHYTOPATHOLOGICAL SOCIETY OF ISRAEL Apri l 2 0 2 1 , 1986 The Vo lcan i Cente r , A R O , Bet Dagan, Israel A: EPIDEMIOLOGY OFPLANTDISEASES IMPROVED "SINGLE TILLERS" METHOD FOR LOSS ASSESSMENT IN WHEAT D. SHTEINBERG*, A. MARANI** and A. DINOOR* *Dept. of Plant Pathology and Microbiology and **Dept. of Field and Vegetable Crops, The Hebrew University of Jerusalem, Faculty of Agrieulture, Rehovot, Israel Common methods for loss assessment include establishment of experimentally derived damage functions expressing the relationships between disease level and yield; and estimates, based on the damage functions, of losses in other fields. There are drawbacks to these methods, such as (i) cost, preplanning and prospects for success of the field trials, and (ii) validity of the functions for other fields. In the "single tillers" method the functions are based on hundreds of tillers. The main drawback to it is the validity of estimates derived from single tillers, for a whole crop. The advantages of the method are that (i) it can be applied simply in any desired situation without preplanning and without allocation of field experiments; (it) the damage function is calculated for each field evaluated; and (iii) it is very inexpensive to operate. Attempts to standardize procedures in order to improve the validity of the method were tested in field trials. Tagging only 300 single tillers was found to be the optimal sample size which provided small enough SE values. Disease severity on fiat leaves at the early milk growth stage was the most important disease factor affecting yields. Different loss models were tested (the critical stage model, the multiple regression model, the stepwise regression model, and the area under disease progress curve model) and were found satisfactory for predicting yield losses. The critical stage model, which is simpler than the rest, was then chosen for use under Israeli conditions. By taking into account plant factors unaffected by the disease, percentage of yield variability explained by disease level was improved from previous analyses explaining only 3-8% of the variation, to new analyses explaining 53-78% of the variation. The losses determined by the improved method did not differ significantly from the values calculated from ordinary field trials. "CRWD-1 AND -2" (DECISION MAKERS 1 AND 2): COMPUTERIZED RECOMMENDATION SYSTEMS FOR THE CONTROL OF FOLIAR WHEAT DISEASES D. SHTEINBERG*, A. DINOOR* and A. MARANI** *Dept. of Plant Pathology and Microbiology and **Dept. o.f Field and Vegetable Crops, The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot, Israel Foliar wheat diseases may develop into epidemics and cause appreciable losses to yields. Epidemic development depends on climatological and agronomical conditions and it is impossible to forecast the specific disease development in each particular field. Decision making, based on the specific agrotechnical, epidemiological and meteorological conditions, is needed for each field. The computerized method "CRWD-I" (Control Recommendations for Wheat Diseases) Phytoparasitica 14:3, t 986 23 7 was designed for use by the farmers. It combines general information from research (built-in information), with specific information for the particular field (information added by the farmer). It offers a strategy of control, based on disease thresholds, the conditions affecting disease development and specific economic conditions. The method was tested in 1985 in two regions (northern Negev and Bet She'an Valley). All recommendations to withhold control and 75% of the recommendations to apply control, proved to be beneficial. Based on these findings, the method was then improved, to include weather forecasts in addition to the consideration of climatic conditions that had prevailed before decision making, which was used in the first version. The "new version, "CRWD-2", will be tested in 1986 in the

The crop protection adviser is constantly faced with the problem when to counsel the farmer to act to mitigate or prevent damage from abiotic or biotic agents, either by strengthening the plant (water, fertilizers) or by checking pests or pathogens by chemical or cultural means. This is an economic decision taken against the background of biological and ecological considerations: will it, or will it not, benefit the farmer and increase his income if he goes into action to minimize risk or save some of his yield, and can he do so without deleterious effects on pest predators or parasites, or on bees, fishes, wildlife etc., and without undue health hazard (Diercks 1984)?

The scope of advisory work in pest and disease management, its professional, legal and organizational framework, and the supporting services it relies on, clearly comprise a great diversity of objectives and approaches. The differences are most pronounced where such work in developed farm economies is compared with that in less developed ones. In considering how far the advisory systems evolved answer the needs for pest control advice, and what developments are to be desired, we shall unavoidably repeat some of the material and views presented in earlier chapters, especially in Chapter 2.6.

The principal aspects of crop pest and disease work in Israel are covered as follows:

Crop protection, in its early stages, has developed as a number of separate scientific disciplines, including entomology, phytopathology, nematology, weed science, etc., which proceeded on parallel paths rather than in unison. In the case of entomology, in particular, this was largely due to the preponderance of medical entomological research. Weed science was just a part of botanical research until close to the 1950’s.

In the detection and diagnosis of nonparasitic disorders, the distribution of symptoms is often the first step. This comprises answers to two questions: are the symptoms distributed at random or in discernible patterns (e.g., along rows)?, and no less important: are symptoms restricted to one or a few plant species or are they more widely distributed over many species.

A wide range of activities is expected from the adviser for pest and disease control, and he or she is constantly faced with ever-changing, sometimes critical situations that demand action.

In considering the overall aims of the adviser’s work, it must be realized that certain difficulties often arise, both against the background of tasks imposed by his employers and in his relation with growers.

ABSTRACTABSTRACTThe distribution of downy mildews, powdery mildews and rusts over the genera of the 15 tribes of Compositae has been surveyed. The proportion of host genera attacked by each of these groups of pathogens is highest in the Lactuceae, but also high in the Cynareae and Heliantheae. The tribe Mutisieae has very few genera with powdery mildews or rusts, and none with downy mildews; the taxonomic position of this tribe appears to warrant re-examination. Of the 269 genera of Compositae recorded as attacked by one or more of the above pathogen groups, many more have rusts alone (99 genera) than powdery mildews alone (32 genera), and 64 are affected by all three groups of pathogens. If a host genus is affected by a downy mildew, it also is affected by a rust and/or powdery mildew. Downy mildews are thus much less likely to be found on species of Compositae that are not hosts of rusts or powdery mildews. Hosts of Bremia are mostly found in tribes in which hosts of Plasmopara are comparatively rare or absent, and vice versa. Hosts of Peronospora are limited to Anthemideae and Lactuceae, and hosts of Basidiophora to Astereae and Heliantheae. No downy mildew hosts have been recorded in genera of Liabeae and Tageteae. Distribution of Puccinia is general, over all tribes. Species of Uromyces are absent from hosts in Lactuceae. Hosts in genera of Senecioneae, Cynareae, and Lactuceae are most likely to be affected by species of more than one genus of Erysiphaceae. Implications of the distribution of the three groups of pathogens for the taxonomic study of their hosts are discussed.Key Words: CompositaePeronosporaceaeErysiphaceaeUredinales