Guess the Pest! Week 17 Answer: Pythium or Phytophthora

David Owens, Extension Entomologist,

Congratulations to Chris Leon for identifying one of two possible causal agents of last week’s fluffy melon. There are two diseases that can cause this: Pythium and Phytophthora. Apparently, it is really important to bring samples back to the lab to have them identified. I am used to being able to snap a slightly fuzzy picture of a lot of bugs and recognize them. So my apologies to Dr. Kate Everts and Dr. Alyssa Koehler for the low quality sample.

Dr. Kate Everts wrote an extensive article in the WCU 2017 on Phytophthora fruit rot, and can be found here:

A UMass article ( states that “Pythium species can survive indefinitely in the soil on various organic substrates or as long-lived, thick-walled oospores. When free moisture is available, sporangia and zoospores are produced. Fruit infection can be by means of vegetative mycelium, sporangia, zoospores, or oospores. Zoospores are attracted by fruit exudates and swim towards the fruit. The pathogen is capable of direct penetration, but wounds enhance infection.” We had a lot of moisture with the tropical storm a couple of weeks ago, and it is possible that the watermelon in question had sunscald which would’ve made it more susceptible.