Powdery mildew of melons
(watermelon, rockmelon and honeydew)
Christine Horlock, DPI's Agency for Food and Fibre Sciences, Horticulture.
Margaret Tuttle McGrath, Department of
Plant Pathology, Cornell University, NY, USA.
Figure 1 Powdery mildew infected rockmelon leaves.
Powdery mildew diseases affect a large number of different types of plants, eg: roses, tomatoes and cereals as well as cucurbits. For most of these plant types there is a specific powdery mildew fungus, ie: the disease organism causing powdery mildew on roses is different from the one causing powdery mildew on cucurbits. Powdery mildew of cucurbits (including melons) is caused by the fungus Podosphaera xanthii (Px; previously known as Sphaerotheca fuliginea). This fungus is made up of several races, some of which attack all cucurbits while others have a host range restricted to certain types of cucurbits. A specific race which affects watermelon has been identified, and is known as Podosphaera xanthii f. sp. citrullus (Pxc). The exact population of Px versus Pxc in Australia is not known. At this stage there is no significant difference between Px and Pxc with regard to management of powdery mildew in the field.
Symptoms
Leaves
Small, circular, white, powdery patches occur on leaves (Figure 2), runners and petioles (leafstalks). The powdery growth usually develops first on crown leaves, on shaded lower leaves, and on leaf undersurfaces. Typically symptoms begin to develop as plants start to produce fruit. Yellow spots may form on upper surfaces opposite powdery mildew colonies on the lower surface. In the absence of control measures, both surfaces of leaves can become covered with the white powdery growth. Affected leaves whither and die, gradually turning yellow, then becoming brown and papery. Whole plants may senesce prematurely. Occasionally the white powdery growth is found on the stems and petioles only, with no visible symptoms on leaves.
Figure 2. Powdery mildew on the lower and upper surface of watermelon leaves.
Fruit
Fruit infection is rare in watermelon.
Premature loss of leaves can result in reduced market quality in all melon types because fruit become sunburnt or ripen prematurely or incompletely. Such fruit have low soluble solids with consequent poor flavour. Stress from disease can lead to imperfections on fruit rind such as speckling, raised indentations, and oedema. Quality of yield can be reduced due to a decrease in the size or number of fruit or a decrease in the length of the harvest period.
Source of infection and spread
Powdery mildew usually occurs when plants are under some kind of stress, such as the physiological stress associated with fruiting. Stress caused by infection of the plant by other pathogens (particularly virus) can also lead to more severe powdery mildew outbreaks. In addition, powdery mildew predisposes plants to other diseases, in particular, gummy stem blight.
The white mildew consists of large numbers of spores (Figure 3) which are spread by wind. Unlike downy mildew, this disease flourishes in comparatively dry weather, with germination of spores being inhibited by free water on the leaf surface. Light dews provide sufficient moisture for infection. Px occurs as several races, most of which can infect all cucurbits* but some are restricted to one species. Strains resistant to systemic fungicides also occur within the pathogen population.
*With the exception of resistant varieties. Powdery mildew resistance in rockmelons is specific to particular pathogenic races of the fungus. Check with seed companies for information on the nature of powdery mildew resistance in their varieties.
Figure 3. Scanning electron microscope image of powdery mildew spores
on the leaf surface.
Disease cycle
The powdery mildew fungus is an obligate parasite and cannot survive in the absence of a living host plant, except in special spore protecting structures called cleistothecia. Although powdery mildew occurs on a broad range of crops, each species is relatively host specific. Although at this stage non-cucurbit hosts are not thought to play a role in the disease cycle, the significance, if any, of non-cucurbit hosts in generating powdery mildew spores able to infect cucurbits, remains unclear.
The initial source of powdery mildew infection can be difficult to determine as spores are able to travel long distances in the wind. Possible sources include crops grown earlier in the season, spores stored in cleistothecia and spores from alternate hosts.
Powdery mildew develops quickly under favourable conditions, with the length of time between infection and the appearance of symptoms usually only 3-7 days, with a large number of spores being produced in a short time. Favourable conditions for infection and spore survival include dense plant growth, low light intensity and high relative humidity. However infection can take place at relative humidity levels as low as 50%. Dry conditions are favourable for colonisation, sporulation and dispersal. Rain and free moisture on the plant surface are unfavourable for disease development, but it may still occur in the presence of dew. Infection can occur at temps of 10-32oC (mean temps of 20-26oC are favourable). Powdery mildew is arrested at daytime temps of 38oC or higher.
Importance
Powdery mildew is a common and serious disease, occurring regularly in melon fields throughout Australia. Early leaf loss can result in sunburnt fruit in rockmelon crops.
Powdery mildew is also important due to the relative ease with which fungicide resistant strains can develop in affected fields. Work in the US has shown that fungicide resistant populations of powdery mildew can become the predominant type in a field within one growing season.
Control
-
Grow resistant varieties
-
Apply both systemic and protectant fungicides, alternating among systemic fungicides in at least 2 chemical classes and including a protectant fungicide in at least every other application. Do not apply systemic fungicides curatively. Instructions are included on fungicide labels.
Application of fungicides continues to be the principle practice for managing powdery mildew in most cucurbit crops. Resistant varieties are being developed and are becoming an increasingly important component of management programs. However, there is the danger that strains may develop which overcome the resistance gene in the plant. For this reason we recommended an integrated program of fungicides and resistant varieties. Additionally, fungicides will continue to be needed for other diseases.
Powdery mildew needs to be controlled on both leaf surfaces to avoid premature death of leaves. It is especially important to control powdery mildew on the underside of leaves where conditions are more favourable for disease development than on upper surfaces. The best approach is to use systemic fungicides. Advances are being made in sprayer technology to deliver more spray material to the underside of leaves.
**Chemically resistant races of powdery mildew have been isolated in Australia. **
Managing fungicide resistance is an essential component of effective powdery mildew control. Most systemic fungicides are at risk for resistance development because they have single-site mode of action. Thus modification of one gene in the pathogen may be enough to enable the pathogen to resist the action of the fungicide. Most protectant fungicides are not at risk for resistance development because they have a multi-site mode of action. The powdery mildew fungus has demonstrated a high potential for developing resistance. It has already developed resistance in Australia or elsewhere to at least one material in the at-risk chemical classes. Presence of resistant strains has been associated with control failure. It is important to realize that the primary goal of resistance management is to delay its development rather than to manage resistant strains after they have been selected; therefore, a management program needs to be implemented before resistance develops to be effective.
The fungicide program should include both systemic and protectant fungicides. The ideal program consists of an alternation among effective systemic fungicides in at least 2 chemical classes and includes a protectant fungicide with the systemic fungicide in at least every other application. Protectant fungicides are important for resistance management because they control both sensitive and resistant strains of the pathogen. Systemic fungicides should not be applied curatively. Begin applications when powdery mildew just starts to develop (e.g. symptoms found on 1 of 50 older leaves) or start earlier if scouting is not being done. It is critical to examine the underside of leaves for symptoms, especially where protectant fungicides were applied earlier in the season.
Further information
Links to other melon disease DPI notes
Bacterial Fruit Blotch of Melons (watermelon, rockmelon and honeydew)
Fusarium Wilt of Melons (watermelon, rockmelon and honeydew)
Sudden Wilt of Melons (watermelon, rockmelon and honeydew)
Viruses affecting Melons (watermelon, rockmelon and honeydew)
Key contacts
Heidi Martin
Plant Pathologist, Horticulture
Agency for Food and
Fibre Sciences
Department of Primary Industries
Gatton Research
Station
Locked Bag 7
MS 437
Gatton Qld 4343
Phone: (07)
5466 2250
Fax: (07) 5462 3223
Email: heidi.martin@dpi.qld.gov.au
Chrys Akem
Plant Pathologist, Horticulture
Agency for
Food and Fibre Sciences
Department of Primary Industries
Ayr Research
Station
P.O Box 591, Little Drysdale St.
Ayr, Qld 4897.
Phone: (07)
4783 0411
Fax: (07) 4783 3193
Email: chrys.akem@dpi.qld.gov.au
Telephone the DPI Business Information Centre on 13 25 23 (Queensland residents) for the cost of a local call between 8 a.m. and 6 p.m. weekdays; non-Queensland residents phone 07 3404 6999; email callweb@dpi.qld.gov.au
Infopest CD for current registered pesticides: email infopest@dpi.qld.gov.au
Margaret Tuttle McGrath may be contacted by email: mtm3@cornell.edu
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
File No: H0237
Last reviewed 21 January 2004
