Podosphaera xanthii (Castagne) U. Braun & Shishkoff 2000

Watermelon leaf showing an even distribution of powdery mildew (Podosphaera xanthii) over the entire leaf surface. June 1995 Photo by Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org
Watermelon leaf showing an even distribution of powdery mildew (Podosphaera xanthii) over the entire leaf surface. June 1995.  Photo by Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org
California Pest Rating for
Podosphaera xanthii (Castagne) U. Braun & Shishkoff 2000
Pest Rating:  C

Initiating Event:

On July 13, 216, diseased leaves of Calibrachoa sp. plants exhibiting powdery mildew symptoms were collected during a regulatory nursery inspection, from a nursery in San Luis Obispo County, by San Luis Obispo County officials and sent to the CDFA Plant Pathology Laboratory for diagnoses.  Suzanne Latham, CDFA plant pathologist, identified the powdery mildew pathogen, Podosphaera xanthii, as the cause for the disease.  The pathogen was assigned a temporary “Z” rating as it has been reported earlier in California and is considered widely distributed.  That rating is reassessed here and a permanent rating is proposed.

History & Status:

Background: Podosphaera xanthii causes powdery mildew disease primarily in cucurbits under field and greenhouse conditions.  The pathogen has a wide host range which includes several ornamental plants, including Verbena. There are several pathogenically distinct races of P. xanthii, and plant resistance-breaking races are present in California.  The pathogen is widely distributed throughout the world (see ‘Worldwide Distribution’ below) as well as in California.  Within California, powdery mildew may be common in coastal and desert production areas, but is more common in fall in the San Joaquin Valley and Sacramento Valley (Davis, et al., 2012).

Podosphaera xanthii was previously known by several names including Sphaerotheca fuliginea and S. fusca.  In 2012, P. caricae-papayae was synonymized with the morphologically similar species P. xanthii (Braun & Cook, 2012) however, the synonymy of P. xanthii and P. caricae-papayae is in question given the molecular work of Takamatsu et al., (2010) who also inferred that further molecular and morphological studies would help to determine the correct taxonomic position of P. caricae-papayae within the genus Podosphaera in the family Erysiphaceae of the order Erysiphales.  (The CDFA risk assessment and rating for P. caricae-papayae is published separately.)

Hosts: Podosphaera xanthii has a wide host range which includes agricultural crops, ornamentals, few fruit, and weed plant species within eight or more families. Hosts include, Citrullus lanatus (watermelon), Cucumis melo (melon), C. sativus (cucumber), Cucurbita (pumpkin), C. maxima (giant pumpkin, C. moschata (pumpkin), C. mixta (pumpkin), C. pepo (synonym: C. ovifera; ornamental gourd), C. vulgaris, Lagenaria siceraria (bottle gourd), Luffa acutangula (angled luffa), L. aegyptiaca (loofah), L. cylindrica, Momordica charantia (bitter gourd), M. cochinchinensis (gac), Sechium edule (chayote), Cyamopsis tetragonoloba (guar), Phaseolus aconitifolius (synonym: Vigna aconitifolia; moth bean), P. coccineus (runner bean), P. vulgaris (common bean), Vigna mungo (black gram), V. radiata (mung bean), V. umbellata (rice-bean), V. unguiculata (synonym: V. catjang; cowpea), Cajanus cajan (pigeon pea), Sesamum indicum (sesame), Capsicum frutescens (chili), Solanum melongena (eggplant),  Hibiscus mutabilis (cottonrose), H. syriacus (shrubby althaea), Hoheria lyallii (synonym: H. populnea; lacebark), Malva pusilla (round-leaved mallow), Carica papaya (papaya), Coriaria arborea (tree tutu), Kalanchoe blossfeldiana (flaming katy), Petunia x hybrid, , Verbena bonariensis (purpletop vervain), V. brasilensis (Brazilian verbena), V. canadensis (clump verbena), Verbena x hybrida (synonym: V. hortensis; garden verbena), V. incisa, V. lasiostachys (western vervain), V. litoralis (blue vervain),V. macdougalii (MacDougal verbena), V. officinalis (common verbena), V. phlogiflora, V. rigida rigid verbena), Calendula sp. C. arvensis (field marigold), C. officinalis (pot marigold), C. palaestina (Palestine marigold),  Cosmos bipinnatus (garden cosmos), Calibrachoa sp.,  Cephalotus follicularis (Albany pitcher plant), Farfugium japonicum (leopard plant), Glandularia pulchella (South American mock vervain), Gynostemma pentaphyllum (jiagulan), Gerbera jamesonii (African daisy), Gynura bicolor (Okinawan spinach), Helianthus annuus (sunflower), Heteropogon contortus (black speargrass), Jatropha gossypiifolia (bellyache bush), Ligularia sibirica, Medusagyne oppositifolia (jellyfish tree), Melampyrum nemorosum (wood cow-wheat), Melothria japonica (Japanese wild cucumber), Melothria sp. Parasenecio hastatus subsp. tanakae, Pericallis cruenta (common cineraria), Physalis alkekengi (Chinese lantern), Pisum sativum (pea), Pulicaria dysenterica (meadow false fleabane), Senecio chrysanthemoides , S. grahamii, S. hercynicus, S. nemorensis, Impatiens hawker (New Guinea impatiens), Ageratum conyzoides (billy goat weed), Bidens bipinnata (Spanish needles), B. cernua (nodding beggarticks), B. frondosa (devil’s beggartick), B. pilosa (blackjack), B. tripartita (three-lobe beggarticks), Boehmeria nivea (Chinese grass), Buddleja brasiliensis, B. salviifolia, Xanthium californicum (synonym: X. strumarium var. canadense; Canada cocklebur), Xanthium italicum, X. pensylvanicum, X. spinosum, X. strumarium (common cocklebur), X. spinosum (spiny cocklebur) Zinnia elegans (zinnia), (CABI, 2016; Farr & Rossman, 2016).

Symptoms:  Powdery mildew symptoms on cucurbits: the disease first appears as pale yellow spots on stems, petioles, and leaves.  These spots enlarge as white powdery fungal growth comprising primarily of asexual spores (conidia) develops on upper and under leaf surfaces, petioles, and stems of infected plants, usually developing first on crown leaves, shaded lower leaves, and leaf undersurfaces.  Affected leaves become dull, chlorotic and may wilt and eventually turn brown and papery (Davis et al., 2012).  Older plants are initially infected and infected leaves usually wither and die.  Premature senescence may occur.  Fruit infection occurs rarely in cucumber and watermelon.  Minute dark brown chasmothecia (sexually produced, closed fruiting structures) have been rarely observed in infected cucurbits in the USA and may be easily overlooked. They may develop late in the season, and the sexual spores within the structures are protected from adverse conditions.  Symptoms are less common on cucumber and melon as many commercial cultivars are resistant to the pathogen (McGrath, 2011).

Disease cycle:  Podosphaera xanthii is an obligate parasite.  Primary sources of inoculum include conidia which can be dispersed over long distances and remain viable for 7-8 days.  The fungus grows on the surface of plant tissue and invades by sending feeding organs (haustoria) into the plants epidermal cells only in order to obtain nutrients.  Mycelium produces conidiophores on the plant surface.  Each conidiophore produces chains of conidia (spores) that are dispersed by air currents.  Powdery mildew develops quickly under favorable conditions of dense plant growth and low light intensity.  High relative humidity is favorable for infection and conidial survival, and infection can occur as low as 50% relative humidity.  Dry conditions favor colonization, sporulation, and dispersal, however, rain and free moisture on plant surfaces are unfavorable.  Optimum temperature for disease development is 20-27°C with infection occurring at 10-32°C.  Powdery mildew development is arrested at 38°C and higher temperatures.  As an obligate parasite, P. xanthii requires living host plants for survival, however, it may also survive as chasmothecia which have been reported rarely in the United States (CABI, 2016; McGrath, 2011).

Damage Potential:  Powdery mildew can diminish the photosynthetic regions of cucurbit leaves. Severely infected leaves turn brown and shriveled.  Fruit quality and yield are reduced.  In squash, fruit quality is reduced by sunscald and premature ripening resulting in poor storability, in melon, incomplete ripening and poor flavor occurs, and pumpkin fruit may be shriveled and discolored (CABI, 2016).  Late fruit usually fail to mature and are small and misshapen.  Stress from disease can result in speckling and oedema on fruit rind.  Powdery mildew-infected plants can be weakened and predisposed to other diseases.  Podosphaera xanthii can be a major production problem of cucurbits in field and greenhouse conditions (McGrath, 2011).

Transmission:  Conidia (spores) are airborne and dispersed by wind currents.  Laboratory studies have shown that conidia remain viable for 7-8 days. On cucurbits in greenhouses conidia are released and spread from plant to plant via irrigation or air currents.  Conidia can overwinter on cucurbit plants in a greenhouse and then be dispersed from greenhouse to field crops during spring and summer.  Non-cucurbit hosts are not a major source of inoculum due to pathological specialization (CABI, 2016; McGrath, 2011).

Survival:  During cool weather, conidia production ceases and powdery mildew fungi overwinter as chasmothecia and mycelium in weeds and dormant plant tissue. However, the sexual (teleomorph) stage has only been found in California greenhouses and reported for the first time in greenhouse grown squash and melons in Salinas, California (Ramos et al., 2010-2011).  Constant greenhouse growth conditions could perpetuate the asexual stage of the fungus.

Worldwide Distribution: Asia: Armenia, Azerbaijan, Bangladesh, China, Republic of Georgia, India, Iran, Iraq, Israel, Japan, Republic of Korea, Kyrgyzstan, Lebanon, Myanmar, Oman, Pakistan, Saudi Arabia, Singapore, Taiwan, Turkey, Turkmenistan Uzbekistan, Vietnam; Africa: Egypt, Ethiopia, Libya, Somalia, South Africa, Sudan, Tunisia; North America: Canada, Mexico, USA; Central America and Caribbean: Cuba, Puerto Rico; South America: Argentina, Bolivia, Brazil, Nicaragua, Uruguay, Venezuela; Europe: Austria, Bulgaria, Czech Republic, (former) Czechoslovakia, Denmark, Estonia, Finland, Former USSR, Romania, France, Germany, Greece, Hungary, Ireland, Italy, Lithuania, Netherlands, Norway, Poland, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, United Kingdom, Ukraine, Yugoslavia (Serbia and Montenegro); Oceania: Australia, New Zealand, Samoa (CABI, 2016; Farr & Rossman, 2016).

Official Control: Podosphaera xanthii is on the ‘Harmful Organism List’ for Guatemala (USDA-PCIT, 2016).

California Distribution: San Luis Obispo, Solano, and Yolo (CDFA Pest and Damage Records); San Joaquin Valley and Sacramento Valley counties, coastal and desert cucurbit production areas (Davis, et al., 2012).

California Interceptions None reported.

The risk Podosphaera xanthii would pose to California is evaluated below.

Consequences of Introduction: 

1) Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California. Score:

– Low (1) Not likely to establish in California; or likely to establish in very limited areas.

Medium (2) may be able to establish in a larger but limited part of California.

– High (3) likely to establish a widespread distribution in California.

Risk is Medium (2) – Powdery mildew of cucurbits Podosphaera xanthii can occur in coastal and desert production regions, and is common in fall in the San Joaquin Valley and Sacramento Valley. Powdery mildew thrives in warm and humid environments. Low light levels, high humidity, and moderate temperature enhance disease development.  Dry conditions favor conidia production and dispersal.  The pathogen is already widely distributed within cucurbit production regions of the state.   

2) Known Pest Host Range: Evaluate the host range of the pest. Score:

– Low (1) has a very limited host range.

– Medium (2) has a moderate host range.

High (3) has a wide host range.

Risk is High (3) – Podosphaera xanthii has a wide host range that includes agricultural crops – primarily cucurbits, ornamentals, few fruit and weed plants.

3) Pest Dispersal Potential: Evaluate the natural and artificial dispersal potential of the pest. Score:

– Low (1) does not have high reproductive or dispersal potential.

– Medium (2) has either high reproductive or dispersal potential.

High (3) has both high reproduction and dispersal potential.

Risk is High (3) – Under suitable climate conditions, airborne conidia are produced in abundance and readily spread by wind currents to non-infected sites.  Within and outside greenhouse environments, the pathogen is capable of rapidly spreading to non-infested host plants as well as other sites where host plants are grown.

4) Economic Impact: Evaluate the economic impact of the pest to California using the criteria below. Score:

A. The pest could lower crop yield.

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural practices.

E. The pest can vector, or is vectored, by another pestiferous organism.

F. The organism is injurious or poisonous to agriculturally important animals.

G. The organism can interfere with the delivery or supply of water for agricultural uses.

– Low (1) causes 0 or 1 of these impacts.

– Medium (2) causes 2 of these impacts.

High (3) causes 3 or more of these impacts.

Risk is High (3) – The pathogen can potentially cause significant losses in plant growth and crop yield. Powdery mildew infections could lower crop yield and value causing significant losses in production – especially with use of protective and eradicative fungicides.  It could result in loss of markets, and change in cultivation practices to prevent the spread of inocula to non-infected, healthy plants.

5) Environmental Impact: Evaluate the environmental impact of the pest on California using the criteria below.

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.

B. The pest could directly affect threatened or endangered species.

C. The pest could impact threatened or endangered species by disrupting critical habitats.

D. The pest could trigger additional official or private treatment programs.

E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact. Score:

– Low (1) causes none of the above to occur.

Medium (2) causes one of the above to occur.

– High (3) causes two or more of the above to occur.

Risk is Medium (2) The powdery mildew pathogen could significantly impact home/urban gardening and/or ornamental plantings.  

Consequences of Introduction to California for Podosphaera xanthii:

Add up the total score and include it here. (Score)

-Low = 5-8 points

-Medium = 9-12 points

High = 13-15 points

Total points obtained on evaluation of consequences of introduction of P. xanthii to California = (13).

6) Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. (Score)

-Not established (0) Pest never detected in California, or known only from incursions.

-Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

-Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Evaluation is High (-3).  Powdery mildew of cucurbits caused by Podosphaera xanthii is widely distributed in California. 

Final Score:

7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: (Score)

Final Score:  Score of Consequences of Introduction – Score of Post Entry Distribution and Survey Information = 10




The assessment of risk of Podosphaera xanthii distinctly documents its economic importance to California agriculture and environment and its widespread distribution within cucurbit production regions of the State.  Due to its biological capacity for rapid spread and its current widespread instate status, it is highly unlikely that the powdery mildew pathogen of cucurbits can be eradicated from California.  However, control measures, including use of protectant fungicides and resistant varieties, have proven successful in significantly reducing disease intensities and spread (CABI, 2016).  Therefore, a ‘C’ rating is proposed for this pathogen.

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Podosphaera xanthii is C.


Braun, U. and R. T. A. Cook.  2012.  Taxonomic manual of the Erysiphales (Powdery Mildews).  Centraalbureau voor Schimmelcultures, vol. 11, 707 p.

CABI.  2016.  Podosphaera xanthii (powdery mildew of cucurbits) full datasheet.  http://www.cabi.org/cpc/datasheet/50922.

Davis, R. M., T. A. Turini, B. J. Aegerter, and J. J. Stapleton.  2012.  Cucurbits powdery mildew pathogens: Sphaerotheca fuliginea (=Podosphaera xanthii) and Erysiphe cichoracearum (=Golovinomyces cichoracearum).  UC IPM Pest Management Guidelines: Cucurbits UC ANR Publication 3445. http://ipm.ucanr.edu/PMG/r116100711.html .

Farr, D.F., and A. Y. Rossman.  2016.  Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA.  Retrieved January 28, 2016, from http://nt.ars-grin.gov/fungaldatabases/.

McGrath, M. T.  2011.  Vegetables: powdery mildew of cucurbits.  Vegetable MD Online, Cooperative Extension, New York State, Cornell University.  Fact sheet Page: 732.30 Date: June 2011.  http://vegetablemdonline.ppath.cornell.edu/

Ramos, C. B., K. Maruthachalam, J. D. McCreight, and R. S. Garcia Estrada.  2010-2011. Podosphaera xanthii but not Golovinomyces cichoracearum infects cucurbits in a greenhouse at Salinas, California.  Cucurbit Genetics Cooperative Report 33-34: 24-28.

Takamatsu, S., S. Ninomi, M. Harada and M. Havrylenko.  2010.  Molecular phylogenetic analyses reveal a close evolutionary relationship between Podosphaera (Erysiphales: Erysiphaceae) and its rosaceous hosts.  Persoonia, 24, 38-48.

USDA PCIT.  2016.  USDA Phytosanitary Certificate Issuance & Tracking System.  https://pcit.aphis.usda.gov/PExD/faces/ReportHarmOrgs.jsp.

Responsible Party:

John J. Chitambar, Primary Plant Pathologist/Nematologist, California Department of Food and Agriculture, 3294 Meadowview Road, Sacramento, CA 95832. Phone: 916-262-1110, plant.health[@]cdfa.ca.gov.


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