Sclerophthora macrospora (Sacc.) Thirum., C.G. Shaw & Naras.

Synonyms

Sclerospora macrospora Sacc.
Kawakamia macrospora (Sacc.) Hara
Nozemia macrospora (Sacc.) Tasugi
Phytophthora macrospora (Sacc.) S. Ito & I. Tanaka
Sclerospora kriegeriana Magnus
Sclerospora oryzae Brizi
Phytophthora oryzae (Brizi) Hara

Description

Sporangiophores not observed on Australian specimens but reported (Waterhouse 1964) to arise from stomata on one or both leaf surfaces, indistinct en masse, 7.5-27.5 x1.25-3.75 µm, hyaline, sympodially branched, each bearing 4-5 sporangia.

Sporangia not observed on Australian specimens but reported (Waterhouse 1964) to be pyriform-obovate-ellipsoidal, 58-98 x 30-65 µm, hyaline-feintly purple, slightly blunted-papillate, germinating by release off zoospores or occasionally by production of a germ tube.

Zoospores not observed on Australian specimens but reported (Waterhouse 1964) as 24-48 per sporangium, ovate-reniform while swimming, globose at rest, 12-16x10-13 µm.

Oogonia formed mostly in the mesophyll surrounding the vascular bundles, also in other plant organs including roots, globose-subglobose, (50-)54-70(-80) µm in diameter, pale cinnamon yellow, wall 3-9 µm thick.

Oospores globose-subglobose, (35-)42-56(-65) µm in diameter, wall 3-7 µm thick closely adherent to the oogonium wall, germinating by the production of a sporangium at the tip of a pedicel equal to or longer than the diameter of the oogonium (not observed).

Hosts
   
Host family: Poaceae
   
Host species:

Anthosachne plurinervis - BRIP *
Avena fatua - VPRI 253
Avena sativa - BRIP *
Avena sterilis subsp. ludoviciana - BRIP
Bromus catharticus - BRIP
Lolium multiflorum - BRIP
Oryza sativa - BRIP , BRIP *
Phalaris canariensis - BRIP *
Phalaris paradoxa - BRIP
Sorghum bicolor - BRIP *,
Triticum aestivum - BRIP , ; VPRI 254, 255, 256, 257, 10832, 15675
X Triticosecale - BRIP *
Zea mays VPRI 17243, 42049


Distribution

NSW, NT, QLD, SA, TAS, VIC, WA

Comments

The first documented report of Sclerophthora macrospora in Australia was in Cooke’s (1892) Handbook of Australian Fungi as #1696 Sclerospora macrospora on leaves of Alopecurus. Saccardo (1890) had described Sclerospora macrospora based on a specimen containing oogoonia in the inflorescences of Alopecurus collected in Australia by the Polish-borne botanist Johann Gottlieb Otto Tepper. The oldest Australian herbarium specimen of S. macrospora is on oats (Avena sativa), collected at Yanco, NSW on 31 November 1924. Since the genus Sclerophthora was erected in 1953 (Thirumalachar et al., 1953) with S. macrospora as the type species, 3 new species have been assigned to it, namely S. rayssiae with the varieties rayssiae and zeae, S. lolii and S. cryophila, differing primarily on the dimensions of sporangia and oogonia (Safeeulla, 1976). Safeeulla (1976) listed over 140 grasses as hosts of S. macrospora.

The ranges in diameter of both the oogonia and oospores in Australian specimens that were examined (* in above list) are narrower than those (oogonia 55-100 x 50-95 µm and oospores 42-73 x 42-70 µm respectively) quoted in Waterhouse (1964) and Safeeulla (1976). However, the ranges of the specimens from Australia fall within the lower end of the ranges for both parameters, with the minimum of the oospore diameter range being less than that quoted by Waterhouse (1964). These oogonia may have been immature.

This pathogen causes a range of symptoms on its many hosts, including “crazy top”, a symptom characterised by twisting and proliferation of the floral elements of inflorescences, stunting, shortened internodes, profuse tillering, development of roots and adventitious buds at the nodes, and failure of leaves to unfurl causing buckling and distortion. By contrast, other Sclerophthora species eg., S. rayssiae on maize do not cause tissue malformations, but rather narrow, chlorotic stripes, later turning red-purple on leaf blades, similar to those caused by many Peronosclerospora species.

Work on Eluisine corocana in India (Safeeulla 1976) has elucidated information on key biological stages of S. macrospora. Infection occurs in waterlogged soil during the seedling stage, when oospores germinate by production of sporangia which release zoospores. After a short period of motility the zoospores encyst and produce a germ tube which invade roots and lower stems and leaves in contact with the soil and floodwater. The first symptoms on that host are chlorotic streaks on the lower leaves, and after the mycelium systemically invades the growing point, later formed leaves become rough and leathery, plants are stunted and new tillers arise from the base of plants. Sporangia are produced at night when temperatures are 23-25°C (E. corocana in India) and 24-28°C (maize in U.S.) and there is a film of water on the leaves.

The pathogen can be seedborne, because Safeeulla (1976) reported mycelium and oospores inside caryopses and oospores in glumes aand other floral parts which enclose the caryopses of many hosts after maturity. However, Smith and Renfro (1999) stated that seedborne transmission in maize is relatively unimportant because the inoculum is short-lived. 

In Australia, outbreaks of downy mildew caused by S. macrospora are uncommon and cause little economic damage because they are restricted to areas which have received above-normal rainfall or areas of paddocks which have been waterlogged.

Highslide JS
Sclerophthora macrospora on Triticum aestivum - BRIP 16500. Scale bar = 1 cm.
Highslide JS
Oospores of Sclerophthora macrospora on Triticosecale sp. - BRIP 16537.
Scale bar = 100 µm.
Highslide JS
Oospores of Sclerophthora macrospora on Triticosecale sp. - BRIP 16537.
Scale bar = 10 µm.