Abstract:
Sugar beet is a sugar-yielding crop that contributes 25% of the global sucrose production.
Economic production of sugar beet is hampered by Cercospora leaf spot (CLS) (Cercospora
beticola), Rhizoctonia crown and root rot (RCRR) (Rhizoctonia solani), Sclerotinia root rot
(SRR) (Sclerotinia sclerotiorum), and Rhizopus root rot (Rhizopus arrhizus) diseases. These
diseases can reduce yield by 15 to 40%. On CLS, buildup of fungicide-resistance strains is a
major issue due to poor implementation of fungicides and understanding of disease
development at early stages. The identification of germplasm resistant to RCRR disease is
hindered by the lack of effective inoculation methods. Identification of SRR and RRR
pathogens is crucial for their proper management. The objectives of this research were 1. To
evaluate the role of adjuvants in improving the efficacy of fungicides on CLS, 2. To
characterize the infection process during early stages of infection by C. beticola, 3. to
identify an effective inoculation method for RCRR, and 4. to identify and characterize the
causal organisms of SSR and RRR. The value of adjuvants was evaluated in greenhouse and
field conditions. Application of fungicides with or without adjuvants before disease onset
reduced disease severity of CLS in greenhouse condition. In field conditions, additions of
adjuvants did not improve the effectiveness of fungicides and few of them negatively
impacted root yield. The initial stage of infection on CLS susceptible and resistant sugar beet
variety were compared using confocal microscopy. C. beticola biomass accumulation,
percent leaf cell death and disease severity were all significantly greater in the susceptible
variety compared to the resistant variety (P<0.05). R. solani inoculated on the crown and
roots were compared in a replicated trial in greenhouse conditions. The root inoculation
method provided a more consistent disease rating of the sugar beet variety in the greenhouse
for screening of RCRR cultivars in a resistance breeding program. Based on morphological
and molecular techniques, causal organisms of SRR and RRR were characterized and were
found to be pathogenic to sugar beet varieties tested in-vitro and in the greenhouse
conditions.
