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Oomycete Diseases of Cucurbits:

History, Significance, and

Management

Mohammad Babadoost
Department of Crop Sciences, University of Illinois, Urbana,
IL, USA

ABSTRACT

Oomycetes were originally classified among the fungi, as Oomycota. It is now
known that oomycetes are allied with certain algae and thus they were excluded
from true fungi and placed in the kingdom Chromista. Oomycetes produce
filamentous hyphae that resemble those of the true fungi, but without cross
septa (coenocytic hyphae). The cell wall in oomycetes is composed primarily of
훽-glucans and cellulose, while those of true fungi are mostly chitin. Their asex-
ual reproduction is by means of sporangia and biflagellate zoospore, and their
sexual spore is an oospore. Oospores are thick-walled, resistant structures capa-
ble of surviving under unfavorable environmental conditions. Major oomycete
pathogens of cucurbits arePhytophthora capsici,Pseudoperonospora cubensis,
andPythiumspp.Phytophthora capsiciis the most importantPhytophthora
species infecting cucurbits and it can infect cucurbit plants at any growth stage
causing damping-off, crown rot, foliar blight, and fruit rot.Phytophthora cap-
siciwas first described in 1922 in New Mexico, USA. Since then, this pathogen
has been reported from many cucurbit-growing areas in the world. It can infect
more than 50 plant species in more than 15 plant families. Main hosts ofP. cap-
siciare cucurbits and peppers.Phytophthora capsiciis a soil-borne pathogen
and survives between crops as oospores in soil or mycelium in plant debris.
Oospores can survive in the soil, in the absence of a host plant, up to 4 years.
Pseudoperonospora cubensisincites downy mildew, a significant disease
of cucurbits. Downy mildew of cucurbits was first reported in 1868 in Cuba.
Now, this disease occurs in most of the cucurbit-growing areas in the world.

Horticultural Reviews, Volume 44, First Edition. Edited by Jules Janick.
© 2017 Wiley-Blackwell. Published 2017 by John Wiley & Sons, Inc.

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