28[7] Cucumber powdery mildew [EIF

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This leaflet is produced by the Ministry of Agriculture & Livestock, Solomon Islands, with support from IPPSI:
Improved Plant Protection in Solomon Islands, a project financed by ACIAR, the Australian Centre for
International Agricultural Research, Canberra. Authors: Helen Tsatsia, MAL & Grahame Jackson, TerraCircle
Inc.

Extension Fact Sheet 63:

Cucurbit Powdery Mildew

Common name: Powdery mildew of
cucurbits

Scientific name: In Solomon Islands, and other Pacific Island countries, only the asexual
state has been found; this is known as Oidium. The Oidium or asexual state of many powdery
mildews looks the same. However, from research elsewhere, it is known that Cucurbit
powdery mildew belongs to one of two sexual states: Sphaerotheca fuliginea or Erysiphe
cichoracearum.

Hosts: Cucurbits: cucumber, melon, pumpkin, squash and zucchini. Some legumes and
ornamentals are also susceptible.

Damage

The disease can be a major problem. Powdery mildews seldom kill their hosts, but use their
nutrients, reduce photosynthesis, impair growth and reduce yields. Yields are reduced
because the size or number of fruit declines, or plants die early. Early death of the leaves
lowers market quality because fruit become sunburned, ripen prematurely, do not store well,
or the flavour is poor. In addition, plants with powdery mildew are more likely to become
infected with other diseases, Gummy stem blight in particular (see Fact Sheet No. 7).

Biology and Life Cycle

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 appears
white (photo, left: cucumber & right: pumpkin). 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

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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.

Signs and Symptoms

Look for white, powdery fungal spots on upper leaf surfaces, petioles and stems. Look for the
fungus on the shaded older leaves. Infection on the under surface is often not as clear because
of the lighter colour of the leaf; although on the pumpkin (photo, right) it is exceptionally
clear. The fungal spots expand and multiply rapidly, so look for leaves that gradually turn
yellow, then brown, dry out and die.

Management

Cultural control:

Cultural control measures are important to reduce the disease:

•  Plant in sunny places, and, if possible, choose areas with good air circulation;
•  Do not plant successive crops of cucurbits in the same gardens;
•  Do not plant new crops next to those that have the disease; otherwise spread of the

disease to the new crop will be rapid and devastating;

• Remove weeds from within and around the crop: many weeds are hosts of powdery

mildew, so make sure they are controlled.

Resistant varieties:

• There are varieties of cucumber and melon with resistance. Check company descriptions

of varieties of pumpkin and squash, and choose those with resistance.

Chemical control:

If fungicides are used, inspect the crop regularly to detect when infections first occur. Start
spraying immediately, and spray routinely, every 7-10 days depending on the severity of the
disease. READ THE INSTRUCTIONS BEFORE USING ANY PESTICIDE.

• If following an organic regime, use products containing horticultural oil, potassium

bicarbonate or wettable sulphur. The last two are best applied before disease symptoms
appear. Oils are eradicants, meaning they can cure plants that are already infected, but
also have some protectant activity.

• Wettable sulphur has been used for many years for powdery mildew control. Apply in the

early morning or evening, at the coolest times of the day. Never spray sulphur within 2
weeks of the last application of an oil spray. Also, sulphur can burn the leaves, so read the
label carefully before applying.

• Where number of plants is small, use milk: normal strength diluted 1 part in 10 parts

water.

• For large commercial plantings, apply wettable sulphur products, chlorothalonil (also

useful for Gummy stem blight control) or check availability of other products (e.g.,
triazoles), and seek advice from MAL extension officers.