Photo 2. White spots of powdery mildew, Oidium species, on the top surface of a pumpkin leaf. The plants appeared in Photo 3.
Cucumber powdery mildew
Podosphaera xanthii and Golovinomyces cichoracearum; previously Sphaerotheca fuliginea and Erysiphe cichoracearum, respectively. These are the names of the sexual forms of the powdery mildews, but in Pacific island countries only the asexual form has been found, known as Oidium species. Unfortunately, unless the Oidium form is examined miscoscopically soon after collecting identification is difficult.
Even in 1981, the Survey of Agricultural Pests and Diseases1 recognised that samples identified as Sphaerotheca fuliginea were likely confused with Erysiphe cichoracearum. Since then other taxonomic complications have arisen from molecular analyses. Golovinomyces cichoracearum, for instance is recognised as a "species complex", i.e., it is more than one species.
In the interim, and while the taxonomy is better understood, it is best to refer to all powery mildew fungi of cucurbits from Pacific islands as Oidium species.
Worldwide. CABI records Golovinomyces cichoracearum from Asia, Africa, North, South and Central America, the Caribbean, Europe, Oceania2. It is recorded from American Samoa, Australia, Fiji, New Caledonia, and New Zealand. Podosphaera xanthii is recorded from Asia, Africa, North and South America, the Caribbean, Europe, Oceania. It is recorded from Australia, New Zealand and Samoa.
Cucurbits are hosts; common cucurbits are cucumber, melon, pumpkin, squash and zucchini. Some legumes and ornamentals are also susceptible to infection.
The fungus is unusual in that it cannot survive in the absence of a living host. It is also unusual in that it grows over the leaf surface, and that is the reason why the leaf (above and below) appears white (Photos 1-3). In order to feed, the fungus sends special cells into the leaf and these extract the food it needs for growth. Spores, called 'conidia', are produced in chains on single strands of its cottony growth that stand erect from the leaf surface. The spores are spread in the wind; they need high humidity to germinate, but not water. The time between infection and symptoms is short, 3-5 days (depending on temperatures). Spread is fast, especially if plants are close together, humidity is high, and there is no rain.
The disease can be a major problem. Powdery mildews seldom kill their hosts, but use their nutrients, reduce photosynthesis, impair growth and reduce yields. Early death of the leaves lowers market quality because fruit become sunburnt, ripen prematurely, do not store well, or have poor flavour. The size and number of fruits is also reduced. In addition, plants with powdery mildew are more likely to become infected with other diseases, gummy stem blight in particular (see Fact Sheet No. 07).
Look for white, powdery fungal spots on upper leaf surfaces, and on petioles and stems. Look for the fungus on the shaded older leaves. Infection on the underside is often not clear because of the lighter colour of the leaf; although on pumpkin leaves this is not always so (Photo 3). The fungal spots expand and multiply rapidly, so look for leaves that gradually turn yellow, then brown, dry out and die.
Cultural control measures are important to reduce the disease; do the follow:
There are varieties of cucumber, melon, pumpkin and squash with tolerance to powdery mildew. Check company descriptions of varieties, but note that the varieties may not be tolerant to the powdery mildew species in the Pacific.
If fungicides are used, inspect the crop regularly to detect when infections first occur. Start spraying immediately symptoms are seen, and spray routinely every 7-10 days, depending on the severity of the disease. Do the following:
AUTHORS Helen Tsatsia & Grahame Jackson
1Dingley JM, Fullerton RA, Mckenzie EHC (1981) Survey of Agricultural Pests and Diseases Technical Report Volume2. South Pacific Bureau for Economic Co-operation, United National Development Programme, Food and Agricultural Organization of the United Nation. Rome, Italy. 2CABI (2017) Golovinomyces cichoracearum (powdery mildew) Crop Protection Compendium. (www.cabi.org/cpc). CABI (2016) Podosphaera xanthii (powdery multdew of cucurbits) Crop Protection Compendium. (www.cabi.org/cpc). Photo 4 (taken by Eric McKenzie), and used in this fact sheet, appeared previously in McKenzie E (2013) Golovinomyces cucurbitacearum PaDIL - (http://www.padil.gov.au).
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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