Sugar beet (Beta vulgaris L.) is primarily used for the production of
sugar, however, the yield is largely affected by below-ground diseases.
In April 2018, sugar beet stecklings demonstrated chlorotic foliage in
the seed production area of Arizona (34.0489° N, 111.0937° W). Roots
appeared water soaked lesions. The disease incidence was about 2%.
Beet roots were washed to remove soil particle, surface-sterilized in a
10% NaOCl solution for 1 minute, and dipped twice in sterile water.
Isolations were done on MBV (metalaxyl-benomyl-vancomycin, 72 h
at 25 ± 2°C) selective media [1]. It yielded to oomycete-like organisms
that were transferred to CMA (Corn Meal Agar, 14 days, 25 ± 2°C)
using hyphal-t... More
Sugar beet (Beta vulgaris L.) is primarily used for the production of
sugar, however, the yield is largely affected by below-ground diseases.
In April 2018, sugar beet stecklings demonstrated chlorotic foliage in
the seed production area of Arizona (34.0489° N, 111.0937° W). Roots
appeared water soaked lesions. The disease incidence was about 2%.
Beet roots were washed to remove soil particle, surface-sterilized in a
10% NaOCl solution for 1 minute, and dipped twice in sterile water.
Isolations were done on MBV (metalaxyl-benomyl-vancomycin, 72 h
at 25 ± 2°C) selective media [1]. It yielded to oomycete-like organisms
that were transferred to CMA (Corn Meal Agar, 14 days, 25 ± 2°C)
using hyphal-tip methods [2]. Ten isolates consistently showed white
coenocytic mycelia and spherical Oogonia [3]. DNA was extracted
from four isolates using a NORGEN BIOTEK CORP protocol
(Cat.27300). Isolates were confirmed via sequencing (GenScript,
Piscataway, NJ) using the internal transcribed spacer (ITS1/ITS2).
Basic Local Alignment Search Tool (BLAST) demonstrated that the
714-bp sequence was 100% identical to Aphanomyces cochlioides
(GenBank accession no. MK326481.1). Annotated DNA sequence has been deposited into GenBank as MN365601.1. Pathogenicity
experiment was conducted with zoospores culture (prepared from
50% PDA, mixing with autoclaved lake water) and inoculated
in 1-week and 3-week old seedlings (susceptible variety without
any Tachigaren/Hymexazol treatment) in a moist condition. Five
sugar beet plants used with two replications for each time points.
Mock-inoculated sugar beet plants were kept as a negative control.
2-week of post-inoculation, post-emergence damping-off was
observed. 7-week of post-inoculation, inoculated sugar beet plants
were observed with similar symptoms as observed in the field. No
symptoms were observed in the mock treatment. The fungus was reisolated from the diseased beet root tissue, as described above and
the isolates further confirmed microscopically to be A. cochlioides.
This pathogen was previously reported in sugar beet to cause seedling
root rot in Colorado, Nebraska, North Dakota, Southern Minnesota,
and Wyoming [4-6]. To our knowledge, this is the first report of A.
cochlioides causing root rot on sugar beet in Arizona.
