Improvement of potato late blight forecasting
Nærstad R1, Le VH1, Hermansen A1, Hannukkala A2, Nielsen BJ3, Hansen JG3, Grönberg L4, Anderson B4 & Yuen J4.
1 Bioforsk, Norwegian Institute for Agricultural and Environmental Research, Plant Health and Plant Protection Division, Høgskoleveien 7 N-1432 Ås, Norway ,
2 Agrifood Research Finland
3 University of Aarhus, Denmark
4 Swedish University of Agricultural Sciences
SUMMARY
A new potato late blight forecasting model based on hourly weather date is developed based on trials with spore traps and trap plants. The model is built up of sub models forfor the different steps in the disease cycle, spore production, spore release, survival and infection of spores. Long humid periods are needed for spore production and at moderate humidity the process goes slower. The amount of viable attached sporangia is reduced by drought,and some spores are washed off during rain. Spores are released into the air by a drop in humidity or increased radiation, but the release is inhibited by high leaf wetness. The amount of viable released spores is strongly inhibited by solar radiation. The spore load is also reduced by precipitation. The leaf wetness duration have to be sufficient for the spores to germinate and infect. The risk of blight development is a function of the amount of viable released spores and the duration of leaf wetness.
KEYWORDS
Phytophthora infestans, spore production, release, survival, infection, forecast.
INTRODUCTION
Potato late blight, caused by Phytophthora infestans, is a considerable problem in most potato production areas (Fry et al. 2001). The most common way to control potato late blight is to apply fungicides. To optimize the effects of the fungicide it should be applied shortly before the infection happens. Development of the potato late blight epidemic is highly dependent on the weather. This has been known for a long time and in Norway potato late blight forecasting started in 1957. The P. infestans population in the Nordic countries has changed in the last decades. In a Nordic project, called NORPHYT, experiments to study the different factors affecting spore production, release and infection was conducted. The gained knowledge from these experiments in addition to accumulated knowledge from potato late blight research and literature studies has been used in the attempt to improve late blight forecasting (Mizubuti et al 2000, Schepers 1998, Hansen 1992).
MATERIALS AND METHODS
In the NORPHYT trials 2006-2008 a Burkhard volumetric spore trap and trap plants were placed in the centre of a potato field infected with potato late blight to catch P. infestans inoculum. Potato plants, of cultivar Bintje, grown in pots in the greenhouse were used as trap plants. One set of trap plants was put out at eight o’clock in the morning and collected at three o’clock in the afternoon and incubated wet (sprayed with water and covered with plastic). The other set of trap plants was put out at three o’clock in the afternoon and collected at three o’clock the following afternoon and incubated dry. Late blight infection on the trap plants were recorded after one week of incubation at 15-18 ⁰C. Spores on the tape in the spore trap were counted in the microscope and recorded per hour. Infection data and spore catches were analyzed in the relationship to the weather data (temperature, relative humidity, rain, wind, leaf wetness duration and global radiation) to find the factors affecting spore production, release and infection.
RESULTS AND DISCUSSION
Three years of spore trap and trap plant trials in Norway were used to make the model. P. infestans needs a long humid period to produce spores. To initialize spore production the water vapour deficiency have to be below 220 Pa (equivalent to 82.1 %, 87.1 %, 90.6% and 93.1% relative humidity at 10, 15, 20 and 25 degrees o C respectively) for a period of minimum 80 hour*degrees. The spore production increases with the duration of the humid period. Normally these humid periods occurred during the night. A small pause in the humid period, as long as the water vapour deficiency remains below 520 Pa, reduces the spore production but it can continue if the humidity increases again. The spores are released by a drop in the humidity in the laminar layer around the leaf, either caused by the sunlight heating the leaves or a reduction in the relative humidity. This drop normally occurred during the morning. The spore catch in the spore trap increased after these drops. Rain washes the spores down and the spore catch was low in periods with rain. If the leaf is wet or there is no drop in the humidity the spores seams to remain on the sporangiophores up to two or three days before they perish. The spores need free water from rain or dew to germinate. For the spores to be able to infect the wet period must be minimum 40 hour*degrees. If the wet period is long a higher proportion of the spores will be able to infect.
On some days more blight was observed on the trap plants that were put out three o’clock in the afternoon and collected at three o’clock the following afternoon (incubated dry) than on those that were put out at eight o’clock in the morning (incubated wet). This happened when spores were released during the morning hours and plants in the field were wet from dew and the dew remained long enough for the spores to infect. The trap plants put out at eight in the morning came from the greenhouse, hence they were dry. Spores landing on these trap plants therefore had to survive until the wet incubation in the afternoon to be able to infect. On sunny days there were nearly no infections on the plants incubated wet. This indicates that survival of the spores must be included in the late blight forecasting model.
Different weather conditions promote the distinct steps in the late blight epidemic cycle. Sun promotes spore release, but inhibits spore survival. Leaf wetness inhibits spore release but promotes spore germination and infection. Late blight will not be a problem if the infection conditions are good at a time when spores are not produced or spores are produced but not viable any more. Every step in the epidemic cycle must be included in the forecasting model and in the right order.
The improved potato late blight forecasting model is based on hourly weather data and predicts the risk of spore production, with subsequent spore release, spore survival and infection, if there is inoculum in the field. For the time being only the algorithm in the model is made. These algorithms will be programmed in a test version to validate the model with field trials. A disadvantage with the model is that it needs leaf wetness duration, and this is not currently available in the prognosis from the meteorological office.
CONCLUSIONS
Spore production happens during long humid periods, which mainly occurs during the nights. Spores are released by a drop in the humidity most often occurring in the morning. If there is dew formation in the haulm that remains a few hours after sunrise the spores can infect. If there is no leaf wetness in the morning, the spores have to survive until the next wet period to be able to do any damage. Survival of spores is strictly restricted by solar radiation. The new potato late blight model predicts the risk of spore production and subsequent spore release, survival and infection. The model should be evaluated with weather prognosis and a leaf wetness prognosis needs to be developed.
REFERENCES
Fry, W.E., H.D. Thurston and W.R. Stevenson, 2001. Late Blight in: Compendium of Potato Diseases, Second Edition. Eds: Stevenson, W.R., R. Loria, G.D. Franc and D.P. Weingartner. APS Press. p 28-30.
Hansen J.G., 1992. Microclimate in Potatoes in relation to Potato Late Blight (Phytophthora infestans (Mont.) de Bary). Tidsskrift for Plantavls Specialserie. Beretning nr. S2180-1992. pp 43.
Mizubuti E.S.G., D.E. Aylor and W.E. Fry, 2000. Survival of Phytophtora infestans sporangia exposed to solar radiation. Phytopathology, 90:78-84.
Schepers H.T.A.M., 1998. Epidemiological parameters in decision support systems for Phytophthora infestans. In:Proceedings of the Workshop on European network for development of an integrated control strategy of potato late blight, Carlow Ireland 1997. Eds: Schepers H.T.A.M. and E. Bouma. PAV-Special Report no 3.p 30-36.