Septoria spp. Infection Model
Septoria Infections are possible at low temperatures whereas temperatures below 7°C might not lead to an infections within 2 days. The optimum temperature of the disease is reached in the area of 16 to 21°C. Infections are possible within a period of high relative humditiy or leaf wetness of 14 hours or longer. To meet the conditions we decided to seperate into models for weak, moderate and severe infections. Weak infections can be given if it is possible for the pathogen to infect the host tissue. This means that weak infections can take place if temperatures are in miniumum and leaf wetness periods are of critical duration. A moderate infection will take place under conditions where most infeciton trials lead to reasonable results and severe infections take place under conditions where the pathogen has optimum conditions for infection.
Infection starts after a rain of 0.5 mm. We decided not to use a model for pycnidia formation. The condition needed for pycnidia formation is assumed to be a period with relative humdity higher than 85%. Pycnidia life time is expected to be 24 hours. In all climates where Septoria tritici has a chance to infect we will find 2 hours fulfilling this conditions at nearly every day arround sun rise.
Infection severity evaluation: To be able to assess the Septoria tritici infection pressure in between stage 10 (first leaf trough coleoptile) and stage 32 (node two at least 2 cm above node 1) and in between 32 and 51 (beginning of heading) we have to assess the severity of infections based on climatic conditions. This assesment is done in a 1 to 5 scale. A severity of 1 is given if the condition for a weak infection is fullfiled and it has rained less than 5 mm, otherwise the corresponding sevirity vlaue will be 2. A severity of 3 is given if a moderate infection is fullfilled and it has rained less than 5mm. If it has rained more than 5 mm during a moderate infection or less than 5 mm during a severe infection a severity of 4 is given.
A severe infection with more than 5 mm of rain corresponds with a severity value of 5.
Septoria tritici disease pressure evaluation: The climate is only one factor desciding about the disease pressure in field. The other two factors are the history of the field and the susceptibility of the variety grown. If we can accumulate the disease severity values from stage 10 to stage 32 to value of 4 we can expect a weak disease pressure by the climate. If this value reaches 6 we can expect a moderate disease pressure and if it reaches 10 we can expect a high disease pressure from the climate. Knowing the susceptibility of the variety and the history of the field will lead us to spray or not on a weak or moderate disease pressure in this situation. Having an accumulated value of 10 may lead to a spray in stage 32 anyhow. Decsion of a spray at a later stage is more depending on the spring climate. If we are able to accumulate the severity values since stage 10 to a value of 6 we can expect a weak disease pressure. If this value reaches 10 we can expect an moderate disease pressure and if this value reaches 15 we can expect a high disease pressure form the climate situation.
In FC we show the Septoria tritici Severity together with the three different infection severities in one graph (see above). Due too rainfall and long leaf wetness periodes conditions for a severe infection by S. tritici have been fullfield on May 14th and 16th. The Severity levels reach the highest value of 5 on May 14th, which means that a high risk for infection is now.
Stagonospora nodorum's infection biology differs in some extend form this of S. tritici but this difference is not big enough for a seperate model. Therfore we suggest to use this model for the whole complex of Stagnospora and Septoria diseases in cereals including S. passerinii. S. tritici and S. passerinii tend to need longer leaf wetness periods than S. nodorum. In areas with a high pressure of S.nodorum infections classifed to a weak giving a severity value of 2 should be treated more serious then in other areas.
For Septoria nodorum a risk model is shown in FC (see above). A high risk was determined at the June 17th and July 7th (100%). Depending on the susceptible plant stage for infection plant protection measurements have to bee taken into account if the risk reach 80% (also see wheather forecast, protective plant protection). If the risk was 100% and an infection was already determined systemic plant protection measurements (curative application) has to be taken to protect the plant.
Henze, Mathias: Entwicklung einer anbauparameter- und witterunsabhängigen Befallsprognose von Septoria tritici, CUVILLER VERLAG, Göttingen 2008
B.M. Cunfer: Stagonospora and Septoria Pathogens of Cereals: The Infection Process, in Septoria and Stagonospora Diseases of Cereals: A Compilation of Global Research, M. van Ginkel, A. McNab, and J. Krupinsky editors, Proceedings of the Fifth International Septoria Workshop September 20-24, 1999, CIMMYT, Mexico