Potato Late Blight
Potato Late Blight caused by Phytophthora infestans is one of the most devastating plant diseases. It has led to starvings and emigration when it came to Europe. It is one of the most important diseases and therefore numerous models are available for it. P. infestans is a obligate parasite. It can only live in the green tissue of its hosts. The economic important plants under its hosts are potato, tomato and egg plant. In the cool climate during winter the pathogen will find no green tissue and it has to hibernate in infected tubers or in its fruiting bodies the oospores. Oospores will only be formed in places where two different mating types of P. infestanse are present. This is reported for Europe since the last 25 years. Still of greater importance is the hibernation in infected tubers left as volunteers on the field because of undersize or other reasons or damped on the field as waste form potato storage.
Newer laboratory methods enabled us to check for latent infected tubers in the potato seed. This showed that we have to expect this in the potato seed. The quantities with which we have to expect latent infected seeds is depending on the blight epidemics of the last season in the seed producing area.
P. infestans growth like other oomycetes in the intercellular area of its hosts. Systemic growth is enforced by high relative humidity and by high soil water content or low soil oxygen content. Plants formed by latent or symptomous infected tubers show extended systemic growth in periods with water logging. In the morning during and after such periods you will find potato sprouts covered by white sporangia. Sporangia in oomycetes are formed in the absence of light if relative humidity is high and temperature are high enough. For P. infestans sporangia formation will take place in nights with relative humidity higher than 90% and temperatures warmer than 10°C. Sporangia can be distributed by rain or wind.
In literature we can find informations about sporandia germinating and infecting like conidia. Sporanigia in oomycetes usally germinating with zoospores which are mobile in free water. The zoospores are swimming to stoma trough which its infects its host. Jim Deacon from Institute of Cell and Molecular Biology, University of Edinburgh, found that at temperatures of 12°C and lower the majority of sporangia releases zoospores wheras at temperatures higher than 20°C the majority of the sporangia germinates like conidia with germ tubes. Therefore infection of P. infestans in cool climate is most likely limited by the presence of free moisture which can be given by dew in nights which have more than 90% relative humidity needed for sporangia formation. More sever infections have to be expected with rain distributing the zoospores over the potato field and leading to an expotential increase in infected plants.
In heavily infected plants the pathogen will grow systemic into all plant organs including the tubers. In situations with severe disease pressure the potato leaf has to be killed with herbicide to avoid the infections of the tuber.
In FieldClimate.com the models NOBLIGHT and FRY are implemented