November 26, 2004
Source:  American Phytopathological Society, Plant Disease Notes [edited]

Occurrence of Podosphaera xanthii Race 2 on Cucumis melo in Brazil
R. F. Kobori, O. Suzuki, R. Wierzbicki, and P. T. Della Vecchia, Sakata Seed Sudamerica Ltda., C. P. 427, CEP - 12906-840, Braganca Paulista, SP, Brazil; and L. E. A. Camargo, Escola Superior de Agricultura Luiz de Queiroz, Universidade de Sao Paulo, 13418-900, Piracicaba, SP, Brazil. Plant Dis. 88:1161, 2004; published on-line as D-2004-0722-02N, 2004. Accepted for publication 6 Jul 2004.

Powdery mildew is an important disease of melons (_Cucumis melo_ L.) cultivated in greenhouses in Brazil. Currently, there are 5 races of _Podosphaera xanthii_ (formerly known as _Sphaerotheca fuliginea_) and 2 races of _Golovinomyces cichoracearum_ (formerly known as _Erysiphe cichoracearum_) described on melons worldwide, but only race 1 of _P. xanthii_ has been reported in Brazil (1).

However, typical whitish powdery fungal growth was observed on an experimental hybrid yellow melon resistant to race 1 of _P. xanthii_ during the summer of 2000 in a greenhouse in Braganca Paulista, State of Sao Paulo.

Conidia collected from diseased leaves were spread onto 0.5 percent water agar medium and maintained at 22 deg C for 24 h with 12 h of light and 12 h of darkness. Most of the germinated conidia displayed fibrosin inclusion
bodies when observed in a solution of 3 percent potassium hydroxide (KOH), and approximately 1 of 50 also displayed forked germ tubes. These features allowed us to identify _P. xanthii_ as the causal agent.

Conidia raised on the susceptible yellow melon 'Amarelo CAC' were used to inoculate cotyledons of the differential melon lines (2) 'Hale's Best Jumbo' (susceptible to races 1, 2, and 3 of _P. xanthii_), 'PMR-45' (resistant to race 1 and susceptible to races 2 and 3), and 'PMR-' 6 (resistant to races 1 and 2 and susceptible to race 3). Inoculations were performed on 10 plants of each differential line and replicated 4 times. The presence or absence of symptoms was evaluated 18 days after inoculation. 'Hale's Best Jumbo' and 'PMR-45' were rated as susceptible while 'PMR-6' was rated as resistant, thus indicating the presence of race 2 of _P. xanthii_ in Brazil.

During field surveys from 2001 to 2003, this race was found on squash (_Cucurbita moschata_), summer squash (_C. pepo_), and melons in Sao Paulo.

References:
(1) F. J. B. Reifschneider et al. Plant Dis. 69:1069, 1985.
(2) C. E. Thomas et al. Cucurbit Genet. Coop. 7:126, 1984.

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[Cucumber powdery mildew, caused by the fungus _Podosphaera xanthii_ [Px] is 1 of the world's most widespread and damaging cucumber diseases. Px consists of several races, some of which attack all cucurbits, while others have a host range restricted to certain types of cucurbits. 997 cucumber (_Cucumis sativus_ L.) accessions from the U.S. National Plant Germplasm System (NPGS) collection were tested for resistance to powdery mildew. Plants were evaluated and classified for their resistance to Px. 3 susceptibility grades were used: susceptible (S), intermediate (I) or resistant (R). 94 of 977 accessions (9.6 percent) contained at least 1 I or R-type plant and 17 of the 20 most-resistant accessions originated in Asia. Disease management depends upon use of resistant cultivars and application of 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. Resistant varieties are being developed and are becoming an increasingly important component of management programs. There is the danger, however, that strains may develop which could overcome the genetic resistance of some cultivars.

Links:
<http://www.dpi.qld.gov.au/horticulture/11644.html>
<http://whalonlab.msu.edu/rpmnews/vol.13_no.2/research/rpm_research.htm>
<http://westernfarmpress.com/mag/farming_new_mildews_melons/>
- Mod.DH]