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Downy and powdery mildews on watermelon in Florida, USA

A ProMED-mail post
http://www.promedmail.org
ProMED-mail is a program of the
International Society for Infectious Diseases
http://www.isid.org

In this posting:
[1] Downy mildew
[2] Powdery mildew

******
[1] Downy mildew
Date: Fri 5 May 2023
Source: Speciality Crop Industry [summ. Mod.DHA, edited]
https://specialtycropindustry.com/downy-mildew-disease-confirmed-in-north-florida-watermelons/

Downy mildew has been confirmed in northern Florida watermelons. The disease could spread easily through the region by the high winds this week. Downy mildew is traditionally known as "wildfire" because of its high risk of damage and movement.

If growers suspect downy mildew, they need to react quickly. The disease has to be confirmed by a diagnostic laboratory. They are being cautioned to consider a spray programme targeted at downy mildew.

[Byline: Clint Thompson]

--
Communicated by:
ProMED

******
[2] Powdery mildew
Date: Fri 12 May 2023
Source: Southeast AgNet [summ. Mod.DHA, edited]
https://southeastagnet.com/2023/05/12/powdery-mildew-confirmed-in-north-florida-melons/

Powdery mildew, a potentially devastating disease, has also been confirmed in northern Florida watermelons, like downy mildew. Due to an early warning for the region from the ongoing detection programme, farmers can make their own assessment of how to react. By getting started early, powdery mildew seems to have been held down somewhat. Growers concerned about powdery mildew are advised to rotate targeting fungicides.

[Byline: Clint Thompson]

--
Communicated by:
ProMED

[Downy mildew
------------
Cucurbit downy mildew (CDM) caused by the fungus-like organism (oomycete) _Pseudoperonospora cubensis_ is a devastating disease of these crops. _P. cubensis_ is found worldwide, but it is usually most damaging in tropical and subtropical areas. Different pathogen strains with variable host preferences (pathovars) seem to exist and, for example, a strain that affects cucumbers may not necessarily affect melons. By affecting the leaves and thus the photosynthetic potential of its host, downy mildew reduces yield and fruit quality; it can eventually kill the host plant. Under favourable conditions, plantings can be infected so rapidly that they appear to have been frosted.

The pathogen is an obligate parasite surviving on cultivated or wild cucurbits and volunteer crop plants; cucurbit weeds may serve as sources of inoculum. The disease is spread by wind, rain splash, and mechanical means. Disease development can occur over a wide temperature range under conditions of high humidity. Control strategies include the use of resistant cultivars as well as applications of protectant and systemic fungicides.

In North America, new strains of CDM with increased virulence have been reported (e.g. ProMED posts 20071023.3448, 20110713.2115).

Powdery mildew
--------------
The 2 most commonly recorded fungi causing cucurbit powdery mildew (CPM) are _Golovinomyces orontii_ (previously _Erysiphe cichoracearum_) and _Podosphaera fuliginea_ (previously _Sphaerotheca fuliginea_). The fungi appear to be present in varying combinations and amounts depending on the area. Different strains exist and can affect a range of cucurbit hosts. White fungal growth develops on both leaf surfaces, petioles, and stems, producing masses of spores leading to a 'powdery' appearance. Fruit may be deformed or discoloured and of inferior quality. Heavy infection leads to reduced photosynthetic ability and decreased plant vigour, which can cause serious yield losses.

The fungi are thought to overwinter primarily on plant debris and glasshouse crops. Spores are spread by wind, water, and mechanical means. Disease management includes fungicides and cultural practices to remove inoculum and improve air circulation in crops. Varieties with different levels of resistance are available for pumpkin and some of the other cucurbit crops.

Maps
USA (with states):
https://www.orangesmile.com/common/img_country_maps/usa-map-0.jpg and
https://promedmail.org/promed-post?place=8710085,212

Pictures
CDM symptoms, photo galleries:
https://blogs.cornell.edu/livegpath/gallery/cucurbits/downy-mildew-o-cucurbits-early-symptoms/ and
https://blogs.cornell.edu/livegpath/gallery/cucurbits/downy-mildew-on-cucurbits-other-symptoms-and-imitators/
CPM on pumpkin leaves:
http://upload.wikimedia.org/wikipedia/commons/5/5d/Powdery_mildew_on_pumpkin_leaves_1.jpg
CPM, photo galleries:
http://vegetablemdonline.ppath.cornell.edu/PhotoPages/Cucurbit/Powdery/PowderyPhotoList.htm and
https://blogs.cornell.edu/livegpath/gallery/cucurbits/cucurbit-powdery-mildew/

Links
Information on CDM:
https://www.ontario.ca/page/downy-mildew-cucurbits,
http://vegetablemdonline.ppath.cornell.edu/NewsArticles/Cuc_Downy.htm,
http://ipm.illinois.edu/diseases/series900/rpd927/, and
https://ag.umass.edu/vegetable/fact-sheets/cucurbits-downy-mildew
Information on CPM:
http://vegetablemdonline.ppath.cornell.edu/factsheets/Cucurbits_PM.htm and
https://ag.umass.edu/vegetable/fact-sheets/cucurbits-powdery-mildew
Fungal and oomycete taxonomy & synonyms via:
https://www.indexfungorum.org/Names/Names.asp
- Mod.DHA]