Development of an index for wheat stripe rust infection

[摘要] English: The main objective of this project was to develop an early warning index for infection of stripe rust (Puccinia striiformis f. sp. tritici) in susceptible cultivars of wheat (Triticum aestivum) for the main wheat growing areas of the R.S.A.Various authors are in agreement that temperature and moisture are the major climaticfactors necessary for infection by stripe rust. Temperature limits from the literature varybetween -4 and 30°C and no cut-off values were available. It was therefore decided torun an experiment to observe infection of stripe rust under conditions of high relativehumidity and a range of temperatures, namely 5°C, 10°C, 15°C and 20°C.Results from the experiment indicated that little or no infection occurred for the 20°Ctemperature level and for exposure time periods of one hour or less. Exceptions occurredwhen sudden drops in temperature with removal of plants from growth chambers to theglasshouse took place. For both incidence and severity, it was found that temperaturesgreater than 22°C inhibited infection. The Pan 3349 cultivar was found to exhibit higherincidence than Karee. The statistical analysis shows significance for severity forExperiment 1 and 2, but not for incidence. Temperatures for Experiment 2 were slightlylower than those in Experiment 1 and could be explained by the presence of the fans inExperiment 2, to allow evaporation of dew from the sensors, allowing the sensors toregister correct readings. The resulting higher moisture conditions in the still air InExperiment 1 could have contributed to the higher severity observed in Experiment 1.Conditions in the laboratory are not the same as in the field and possibilities forinfections at higher temperatures in the field have been reported by Park (1990). Hewarned against extrapolation of results from the laboratory to the field, but it wasnevertheless decided to use the results from the experiment as a basis for the developmentof an index for stripe rust infection of susceptible wheat cultivars in South Africa.Values of 14 day total degree days (TDDI4) were calculated from total degree hours(TDH) acquired from the experiment. TDDl4 for the experiment was calculated bysumming degree days (DD) from inoculation until 14 days and relating them to averageincidence observed on day 14. Two linear regression lines were obtained, one for 5°C 15°C temperature level and the other for the 15 °c - 20°C temperature levels. A cut-offTDDl4 value of 227 where the two lines crossed indicated 67 % cut-off value forincidence. TDDl4 was developed from this information and reads as follows:IfTDDI4> 227, then risk of incidence is low at < 67 %.IfTDDl4 < 227, then risk of incidence is high at > 67 %.TDDl4 was validated by testing on 1996 - 1998 and 1999 - 2000 data. Correct predictionvalues for highest and average incidence observation were 50 % and 29 % for 1996 -1998 data (Table 4.7) and 40 % and 33 % for 1999 - 2000 data (Table 4.8). TDDl4however, was thought to be impractical, so TDD7 was developed by using TDD7 for 7days from a wet period. TDD7 = 128 was found to be 1.77 times less than the value forTDDl4 = 227 and so this value of 128 was used as the cut-off value. TDD7 thus reads asfollows:If TDD7 > 128, then risk of incidence is low at < 67 %.If TDD7 < 128, then risk of incidence is high at > 67 %.TDD7 was validated using the 1999 - 2000 data, with total correct predictions of 53 %and 40 % (Table 4.10). It was therefore decided that TDD7 could be used by the produceras an early warning index, although the index would have to be tested in the field so thatnecessary improvements could be made.It is recommended that research on stripe rust-environment interaction be continued, alsoto include other diseases and the recommendations made in Chapter 2. Another possiblerecommendation would be to plan a refined laboratory experiment with a mechanisticapproach to use a constant temperature data determined model in real life situations.Temperature and RH would vary and the results should be useful to producer, as well asthe researcher. A model suitable for various other diseases as well, could be of greatbenefit when the cost of pesticides and fungicides are taken into account. The successfulapplication of such a model would be of great benefit to all. After all, food is our fuel andsustainable production of high quality foodstuffs is essential to our survival.

[发布机构] University of the Free State