Integrated Control of Rhizoctonia Crown and Root Rot of Sugar Beet with Fungicides and Antagonistic Bacteria

Abstract

Rhizoctonia crown and root rot, caused by the fungus Rhizoctonia solani AG 2-2, is one of the most damaging sugar beet diseases worldwide and causes significant economic losses in more than 25% of the sugar beet production area in the United States. We report on field trials in the years 1996 to 1999 testing both experimental fungicides and antagonistic Bacillus sp. for their potential to reduce disease severity and increase sugar yield in trials inoculated with R. solani AG 2-2. Fungicides were applied as in-furrow sprays at planting or as band sprays directed at the crown at the four-leaf stage, or four- plus eight-leaf stage, while bacteria were applied at the four-leaf stage only. The fungicides azoxystrobin and tebuconazole reduced crown and root rot disease by 50 to 90% over 3 years when used at rates of 76 to 304 g a.i./ha and 250 g a.i./ha, respectively. The disease index at harvest was reduced and the root and sugar yield increased with azoxystrobin compared with tebuconazole. The combination of azoxystrobin applied at 76 g a.i./ha and the Bacillus isolate MSU-127 resulted in best disease reduction and greatest root and sucrose yield increase.

Management of soilborne diseases of beetroot in Australia: a review

“…Crown and root rots were reduced by 50% over 3 years in field trials treated with azoxystrobin (76-304 g a.i./ha) as in-furrow sprays. Band spray applications of azoxystrobin to plant crowns at the 4-leaf stage were demonstrably better than in-furrow applications at planting (Kiewnick et al 2001), adding support to the hypothesis that Rhizoctonia crown rot infection is predominantly initiated through deposit of infested soil into plant crowns during cultivation. Stobulurin fungicides are largely protective in their mode of action, with only weak systemic activity.…”

“…Consequently the superior efficacy of azoxystrobin when applied to plant crowns, the site of infection for Rhizoctonia on beets, compared with application to the furrow, is not surprising. In these same trials, tebuconazole reduced crown and root rots by 90% over the 3 years when applied at 250 g a.i./ha as in furrow sprays (Kiewnick et al 2001). These chemicals warrant further assessment for control of other beet pathogens.…”

“…Under warm soil temperatures (25-33°C), the fungus grows through the soil and infects the plant through its leaves, petioles, crown and roots. The disease occurs in most soil types, but is more severe in moist soils (Rush and Winter 1990;Kiewnick et al 2001), especially in low areas where water collects (Whitney and Duffus 1986). Rain, irrigation water or rain splash play an important role in the distribution and spread of sclerotia, infected plant debris or basidospores of R. solani (Baker and Martinson 1970).…”

“…More recently, the new strobulurin chemical azoxystrobin was effective in reducing R. solani infections in a number of crops including sugar beet (Kiewnick et al 2001;Zens et al 1999) and celery (O'Neill et al 1999). Crown and root rots were reduced by 50% over 3 years in field trials treated with azoxystrobin (76-304 g a.i./ha) as in-furrow sprays.…”

“…are classified by means of hyphal anastomosis reactions between isolates. In sugar beet, anastomosis group (AG) 2-2 is the major AG world-wide, causing crown and root rot (Herr 1991), whereas AG-4 is more commonly involved in damping-off (Kiewnick et al 2001). A study of 107 Rhizoctonia isolates collected from diseased beetroots in New York State revealed that other R. solani anastomosis groups and other Rhizoctonia species may also be pathogenic to beetroot.…”