Late Blight Fungal Biology Discussion Group

Potato Late Blight On-line Workshop
The Fungus and the Disease
Discussion Forums

Biology

Welcome to everyone who wishes to participate in this workshop session on the 'Biology of the Fungus', Phytophthora infestans, the causal agent of late blight of potato and tomato. In this section, you are invited to post comments or questions related to biological aspects of the fungus. For example, asexual reproduction for either the A1 or A2 mating types involves the formation of sporangia (spores) which are produced on sporangiophores. These spores can germinate directly at temperatures above about 15 C but each sporangium can also produce 3-8 zoospores at temperatures below 15 C. Zoosporangia each have two flagella to 'swim' in water films. When the mycelium of two mating types are in contact with each other, the potential exists for sexual reproduction which involves the formation of an oospore.

Didier Andrivon - 01:17am Mar 5, 1997
I'm not quite sure whether this question is for the Biology or Epidemiology section, but here it goes. I've read time and over again that direct germination of the sporangia (through germ tubes) would not lead to infections, and that only zoospores would be infectious. However, we've been conducting a number of lab experiments at room temperature (20-22 Celsius) with sporangial suspensions, without any chilling to release the zoospores, and found consistent and high infection rates. Furthermore, the epidemics classically develop at temperatures that are more in line with direct than indirect germination of sporangia. Finally, encysted zoospores eventually produce germ tubes, so I don't quite catch the reason why direct germination would not lead to infections. I'd appreciate any comments, since this has been puzzling me for a while now!

Sophien Kamoun - 05:54am Mar 16, 1997
Didier:
We have also repeatedly obtained infection with sporangial suspensions. Histochemical stainings of infecting sporangia from a GUS expressing P. infestans strain clearly showed that sporangial germ tubes can form appressoria and directly penetrate potato epidermal cells. However, the overall infection efficiency might be lower with sporangia than with zoospores/cysts.