What is Powdery Mildew
Powdery mildew is a common disease of cucurbits under field and greenhouse conditions in most areas of the world. Although all cucurbits are susceptible, symptoms are less common on cucumber and melon because many commercial cultivars have resistance. This disease can be a major production problem. Quantity of yield is reduced due to a decrease in the size or number of fruit or a decrease in the length of the harvest period. Premature senescence of infected leaves can result in reduced market quality because fruit become sunburnt or ripen prematurely or incompletely. Such fruit have poor storability (winter squash), low soluble solids with consequent poor flavour (melon), poor rind colour (pumpkin) (fig. 1), and shrivelled, discoloured handles (pumpkin) (fig. 1). Stress from disease can lead to imperfections on fruit rind such as speckling (fig. 2), raised indentations, and oedema.
Podosphaera xanthii (previously
known as Sphaerotheca fuliginea and S. fusca) and Erysiphe
cichoracearum are the two most commonly recorded fungi causing cucurbit
powdery mildew. E. cichoracearum was considered to be the
primary causal organism throughout most of the world before 1958.
Today, P. xanthii is found more commonly worldwide. A shift in
the predominance of these two fungi may have occurred or the causal organism
may have been misidentified. P. xanthii is a more aggressive pathogen
than E. cichoracearum. E. cichoracearum may have a lower
temperature optimum since this species is found mainly during cooler spring
and early summer periods and P. xanthii appears to progress most
rapidly during the warmer months. The conidia (spores produced
asexually) of E. cichoracearum and P. xanthii are difficult to
distinguish and cleistothecia, which are sexual fruiting bodies (structures
containing spores produced through sexual reproduction), have been observed
less commonly. Consequently, these fungi have been confused. The
name of the fungus frequently has been reported without valid confirmation.
Criteria for differentiating these fungi using the conidial stage were not
identified until the 1960s. The main criterion used is presence of
fibrosin bodies in conidia of P. xanthii. Based on these
criteria, P. xanthii was found to be the predominant fungus, rather
than E. cichoracearum as previously claimed, in several countries.
During recent surveys E. cichoracearum was found rarely and only at
the start of disease development in
Symptoms and Signs
White, powdery fungal growth develops on
both leaf surfaces, petioles, and stems (fig. 3). This growth is
primarily asexual spores called conidia. It usually develops first on
crown leaves, on shaded lower leaves and on leaf under surfaces. Yellow
spots may form on upper leaf surfaces opposite powdery mildew colonies (fig.
4). Older plants are affected first. Infected leaves usually
wither and die. Plants may senesce prematurely. Fruit infection
occurs rarely on watermelon and cucumber. Cleistothecia are dark
brown, small (diameter of about 0.003 inches) structures that are barely
discernable without a hand lens (figs. 5, 6). They develop late in the
growing season. The sexual spores within these structures are
protected from adverse conditions.
Sources of initial inoculum for powdery
mildew developing in
Powdery mildew develops quickly under favourable conditions because the length of time between infection and symptom appearance is usually only 3 to 7 days and a large number of conidia can be produced in a short time. Favourable conditions include dense plant growth and low light intensity. High relative humidity is favourable for infection and conidial survival; however, infection can take place as low as 50% RH. Dryness is favourable for colonization, sporulation, and dispersal. Rain and free moisture on the plant surface are unfavourable. However, disease development occurs in the presence or absence of dew. Mean temperature of 20-27°C is favourable; infection can occur at 10-32°C. Powdery mildew development is arrested when daytime temperatures are at least 38°C. Plants in the field often do not become affected until after fruit initiation. Susceptibility of leaves is greatest 16 to 23 days after unfolding.
Cultural and Biological Controls
Genetic resistance is used extensively in
cucumber and melon, and has been incorporated into most other cucurbit
crops. Most resistant squash and pumpkin varieties contain one or two copies of the same major resistance gene from a wild
cucurbit. Genetic of resistance is different in cucumber and melon.
Recently a decline in the degree of suppression achievable with resistant
varieties has been detected indicating adaptation in Podosphaera xanthii.
Successive cucurbit plantings should be physically separated or at
least planted up-wind of older plantings because older plants can serve as a
source of conidia. Fungicides containing antagonistic fungi for
biological control have been developed.
( Fungicide Resistance Action Committee )
Numbers and letters are used to distinguish the fungicide groups according to their cross resistance behaviour. The numbers were assigned primarily according to the time of product introduction to the market . The letters refer to P = host plant defence inducers, M = multi-site inhibitors, and U = unknown mode of action and unknown resistance risk. Reclassification of compounds based on new research may result in codes to expire. This is most likely in the U – section when the mode of actions gets clarified. These codes are not re-used for new groups; a note is added to indicate reclassification into a new code.
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