October 6, 2005
From: American Phytopathological Society, Plant Disease Notes [edited]
<http://www.apsnet.org/pd/searchnotes/2005/PD-89-1131B.asp>

First report of White Leaf Spot caused by Pseudocercosporella capsellae on Brassica juncea in Australia
L. Eshraghi, School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, Crawley, W.A. 6009, Australia; M. P. You, Department of Agriculture Western Australia, Baron-Hay Court, South Perth, W.A. 6151, Australia; and M. J. Barbetti, School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, Crawley, W.A. 6009, Australia. Plant Dis. 89:1131, 2005; published on-line as DOI: 10.1094/PD-89-1131B. Accepted for publication 7 Jul 2005.

_Brassica juncea_ (L.) Czern & Coss (mustard) has potential as a more drought-tolerant oilseed crop than _Brassica napus_, and the 1st 2 canola-quality _B. juncea_ cultivars will be sown as large strip trials across Australia in 2005. This will allow commercial evaluation of oil and meal quality and for seed multiplication for the commercial release Australia-wide in 2006.

Inspection of experimental _B. juncea_ field plantings at Beverley (32 deg 6 min 30 sec S, 116 deg 55 min 22 sec E), and Wongan Hills (30 deg 50 min 32 sec S, 116 deg 43 min 33 sec E), Western Australia in September 2004 indicated the occurrence of extensive leaf spotting during _B. juncea_ flowering. Symptoms of this disease included as many as 15 or more grayish white-to-brownish spot lesions per leaf, often with a distinct brown margin. Some elongate grayish stem lesions were also observed as reported earlier for _B. napus_ oilseedrape (1).

When affected materials were incubated in moist chambers for 48 h, abundant conidia typical of _Pseudocercosporella capsellae_ (Ellis & Everh.) Deighton were observed that matched the descriptions of conidia given by Deighton (2) and those on _B. napus_ in Western Australia (1).

5 single-spore cultures from lesions were grown on water agar (WA), where the colonies characteristically produced purple-pink pigment in the agar after 2 weeks growth in an incubator maintained at 20 deg C with a 12-h photoperiod (3). Since agar cultures of _P. capsellae_ rarely produce conidia (3), this observation helped with the
verification of the cultures.

Mycelial inoculum from these cultures was used to inoculate cotyledons of 50 7-day-old plants of _B. juncea_ to satisfy Koch's postulates. Small pieces of mycelia were teased out from the surface of the growing margin of potato dextrose agar (PDA) cultures and inoculated onto both lobes of each cotyledon and plants incubated in
a 100 percent humidity chamber for 48 h within a controlled environment room maintained at 20/15 deg C (day/night) with a 12-h photoperiod.

After 2 weeks, lesions 5 to 8 mm in diameter were observed on the cotyledons. There were no symptoms on control plants that were treated with water only. Lesions on infected cotyledons incubated on moist filter paper for 24 h produced abundant cylindrical conidia showing 2 to 3 septa measuring 42.9 to 71.4 micrometers long and 2.9 to 3.1 micrometers wide. Single-spore isolations from these conidia produced typical _P. capsellae_ colonies showing purple-pink pigments in WA, and dark, compacted, and slow-growing colonies with a dentate margin on PDA.

White leaf spot caused by _P. capsellae_ is an important disease of crucifers worldwide, but to our knowledge, this is the 1st report of _P. capsellae_ on _B. juncea_ in Australia. In Western Australia, _P. capsellae_ occurs on _B. napus_ oilseed rape (1) and in 1956, 1984, and 1987, it was recorded on _B. rapa_, _B. oleracea_, and _B.
chinensis_, respectively (4), and on the same range of _Brassica_ hosts in other regions of Australia.

References:
(1) M. J. Barbetti and K. Sivasithamparam. Aust. Plant Pathol.10:43, 1981.
(2) F. C. Deighton. Commonw. Mycol. Inst. Mycol. Pap. 133:42, 1973.
(3) S. T. Koike. Plant Dis. 80:960, 1996. (4) R. G. Shivas. J. R. Soc. West. Aust. 72:1, 1989

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[_B. juncea_ has advantages over _B. napus_ including more vigorous seedling growth, quicker ground-covering ability, greater tolerance to heat and drought, and enhanced resistance to the blackleg fungus, _Leptosphaeria maculans_. WLS survives on residues of infected plants. Under favourable autumn and winter conditions, it produces wind-borne conidia, which mainly cause leaf lesions. In turn, the conidia produced in these lesions are carried by wind and rain to cause secondary spread of the disease. WLS is also spread from infected seeds and from pieces of infected debris present with the seed.

Optimum temperatures for WLS infection are 13 to 18 deg C, but high moisture levels are necessary for disease development. WLS disease usually develops after periods of high rainfall. It can be found in most plantings, but is not usually a cause of heavy yield losses. WLS can cause complete loss of leaves in highly susceptible varieties, with yield losses as high as 30 per cent if the disease is severe. However, losses are unlikely to exceed 5 to 10 per cent with current Australian cultivars. Nitrogen-deficient crops seem to be more severely affected by WLS.

Disease management is similar to that of blackleg which means using crop rotation and good hygiene. Sow only cultivars expressing very high levels of resistance, and practice crop rotation. All varieties recommended for sowing in Western Australia have good adult plant resistance to blackleg. Canola fields should be in a rotation for at least 3 years to allow diseased residues to decompose and reduce the risk of ascospore infection. In recent years, some growers have successfully reduced this break period, but there is increased risk from this practice. If the seedling stage of crop development (cotyledon to 1- or 2-leaf stage) coincides with heavy airborne spore
discharges from nearby stubbles, even adult resistant varieties can suffer substantial damage. If there is no erosion risk, destroy crop residues after harvest to reduce carryover of the fungus on infected stems.

Graze stubbles heavily to reduce fungus carryover. Canola should be planted as far as possible from previous canola crops to reduce the risk of infection by wind-borne ascospores. Do not plant in areas downwind from old stubbles where the prevailing winds are likely to blow air-borne spores.

Links:
<http://www.aces.edu/department/grain/Bulletin1150.htm>
<http://www.canola-council.org/graystemmgmt.aspx>
<http://www.agf.gov.bc.ca/cropprot/greystem.htm>
<http://www.cropscience.org.au/icsc2004/poster/5/1/3/1275_norton.htm>
<http://agspsrv34.agric.wa.gov.au/agency/pubns/farmnote/1994/F02994.htm>
- Mod.DH]