Summary The effects of soil temperature and soil moisture content on the rate of simazine degradation were measured in the laboratory in soils from sixteen sites located in several different countries. First‐order half‐lives under standard incubation conditions were significantly correlated with clay content, organic carbon content and soil pH in a multiple linear regression. The temperature dependence of degradation was similar in the different soils whereas the moisture dependence showed considerable variation between soils. Persistence of simazine was also measured in the same soils in the field and at live additional sites. Weather records from the different sites for the periods of the Held experiments were used in conjunction with constants derived from the laboratory data in a computer program to simulate persistence in the field. In general, the model overestimated residues in the field. About half of the calculated residues were within 25% of those observed, an accuracy sufficient for practical purposes, but on several occasions the discrepancies between calculated and observed residues were greater than 50%. Possible reasons for the discrepancies and requirements for further experiments are discussed.
In his experiments Yamada (1955) found an incubation period of about 10 days for Puccinia horiana P. Henn. At a temperature of 15 ~ ~ Stahl (1964) found a period of 12 days, while Stark and Stach (1965) concluded that this time can vary from at least 10 days to 8 weeks 1. Little being known about the time of incubation in various conditions, the authors decided to do the pilot experiments reported below. Chrysanthemum plants of cv. 'Indianapolis White G 4' were inoculated on July 13, 1966, by spraying with a mixer-made suspension of sporulating lesions (Zandvoort et al., 1968). Immediately following inoculation the plants were placed in a glasshouse with a minimum temperature of 12 ~ ~ and a maximum temperature of 25 ~ ~ During the first 24 h after inoculation the plants were placed under a black polythene plastic foil. The sixth day after the inoculation the first symptoms of P. horiana became visible. In another experiment, plants of cv. 'Indianapolis White G 4' were inoculated on 27 June by spraying with a mixer-made suspension of spores. The treated plants were placed in glasshouses at four temperatures ranging from 10~ to 30~ For each temperature treatment 20 plants were used. Fig. 1 indicates that the time of incubation was shorter than 12 days at 17~ and 21 ~ 13 days at 10~ while at 30~ no symptoms became visible during 29 days. The development of the fungus at 10~ is definitely slower than 17~ or 21 ~ The number of infected leaves after 29 days suggests an optimal infection at 17 ~ At the field inspection done by the Plant Protection Service we commonly found new infections about 2-4 weeks after the destruction of infected plants, which is an indication for an incubation period of at least the mentioned time. An interesting indicacation of a very prolonged incubation period was found on one well isolated plot. Cuttings, rooted at an infected nursery where the temperature was over 30~ for several hours during some days of the rooting period, had been planted there about 17 June, 1966. After planting in the field there were several days with a high relative humidity and a temperature of 10~176On 5 August many necrotic spots and only one withered teleutosorus were found. One week later, fresh teleutosori were observed on ca. 2 ~ of the plants.
About 1964 in several countries of Western-Europe Puccinia horiana P. Henn. was recorded for the first time in chrysanthemums, Chrysanthemum morifolium. In all cases it could be established that the rust was imported with infected cuttings (Boerema and Vermeulen, 1964;Melder, 1964;Jorgensen, 1964;Baker, 1967). During the field inspection performed by the Plant Protection Service we found outbreaks which could not be explained by dispersal with infected cuttings. We therefore collected information on the dispersal of inoculum.In 1965 we visited a plot with chrysanthemums severely attacked by P. horiana; we also found the symptoms in the adjacent plots (Fig. 1). The following observations were made: (1) Infected plants were almost exlusively found in the sector east of the attacked plot, over a distance of up to 350 m. At greater distances there were no chrysanthemums.(2) The greater the distance from the attacked plot, the less was the percentage of infected chrysanthemum plants. (3) Close to the attacked plot, infection was found all over the plants. At a greater distance from the source, infection was usually confined to the upper parts of the plants. (4) In glasshouses in the same sector, infection was found mainly under the ventilation windows. In 1966 we made the following observations: (1) About 7 August a heavily infected crop in a glasshouse was destroyed. A fortnight later a plant with sporulating sori was found outside the glasshouse in a corner with frequent whirlwinds. (2) In another locality, at a nursery close to an infected crop, a mild infection was found especially at a place outside with whirlwinds. (3) In a third locality, at two nurseries east of an infected crop at distances of about 500 and 700 m, respectively, (Fig. 2), we found an infection at places with whirlwinds. Between the infected crop and the two eastward nurseries, no chrysanthemums were cultivated. (4) In several other places we saw that nearly all infections, clearly unrelated to infected cuttings, were found east of the infected crop. These data show that there is a horizontal gradient of infection from an attacked plot. The gradient tapers out in down-wind direction. This direction usually is east because 124
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