A storage onion cultivar
trial was planted near Quincy, WA on 30 Mar 04 to evaluate the performance
of 46 cultivars in the semi-arid Columbia Basin of Central Washington.
The trial was surrounded by a furrow irrigated commercial onion bulb
crop of the cultivar Tamara. A Stanhay belt planter (Stanhay Webb
Ltd., Lincs, England) was used to plant coated seed of each cultivar.
Each plot consisted of two double rows 30 ft long and spaced 22 in.
apart on a 34 in. raised bed, with seed 4 in. apart within rows. A
5 ft alley separated plots at each end. The plots were arranged in
a randomized complete block design with three replications. The crop
was maintained by the grower using standard cultural practices for
fertilization (160-170-60-32-2 lb/A of N-P-K-S-B applied over the
season), irrigation, and disease, pest, and weed control. Incidence
of plants with iris yellow spot symptoms was rated on 11Aug for the
center 5 ft. of each plot. At the same time, iris yellow spot severity
on each plant was rated as mild (1-3 small lesions), moderate (3-5
lesions), and severe (>5 lesions with the youngest leaves turning
necrotic from coalescing lesions). Bulbs were lifted on 4 Sep and
topped and harvested by hand on 23 Sep. The number and weight of bulbs
were determined for colossal, jumbo, medium, and small bulbs. Total
precipitation and mean temperature for Mar, Apr, May, Jun, Jul, Aug,
and Sep were 0 in. and 46.9F, 0.01 in. and 53.1F, 0 in and 58.4F,
0 in. and 66.5F, 0 in. and 74.7F, 0.03 in. and 73.0F, and 0 in. and
60.6F, respectively.
Hail damage occurred at approximately the three-leaf stage, but seedlings
outgrew the mechanical injury. Iris yellow spot was severe at this
site by late Jul. Similarly, sever outbreaks of iris yellow spot were
observed in several other furrow-irrigated onion bulb and seed crops
within 10 miles of this site, reflecting the rapid increase in incidence
and severity of the disease since it was first observed in Washington
in 2002. A distant gradient in incidence and severity of iris yellow
spot was observed across the trial in Aug and Sep, with the highest
incidence in plots of the first replication located approximately
200 ft from the southeast corner of the field, and the lowest incidence
in the third replication near the northeast corner of the field. This
gradient suggested primary infection was probably from viruliferous
thrips migrating into the crop from adjacent vegetation (radish seed
crop to the south, apple orchard to the southeast, or surrounding
native vegetation), with limited secondary movement of viruliferous
thrips within the crop. All 46 cultivars were susceptible to IYSV
with a mean incidence ranging from 58% for Tioga to 97% for EX 15122.
The mean incidence of plants with symptoms of IYSV infection was <70%
for only five of the 46 cultivars (BGS 196, Golden Spike, PS 5299,
T-433, and Tioga), three of which had no plants with sever symptoms
(Golden Spike, PX 5299, and T-433). In contrast, 13 cultivars had
>90% of the plants with symptoms of IYSV infection (Bandolero,
BGS 194, Copra, EX 15122, Flamenco, Genesis, OLYH 02N2, Redwing, Sabroso,
WEO 4001, SX 7004, T-817, and Vaquero), and all 13 cultivars had some
plants with severe symptoms. The means ± standard deviations
of the incidence of plants with mild, moderate, and severe symptoms
were 39 ± 11, 37 ± 12, and 7± 8%, respectively.
Significant differences in yield were observed among cultivars, with
significant negative correlations between total incidence of plants
with iris yellow spot symptoms and total marketable yield (r = -0.43
at P = 0.0029) as well as percentage jumbo bulbs (r = -0.40 at P =
0.0053, r = -0.44 at P = 0.0025). Accounting for inherent differences
in yield among cultivars by using cultivar as a covariate, the regression
relationship between total marketable yield and incidence of iris
yellow spot was:
Mean marketable yield (t/A) = 33.45 + ß0 (cultivar) –
0.11 X1 – 0.14 X2 – 0.12 X3 (R2 = 0.88, CV = 12.1%), where
X1, X2, and X3 = % plants with mild, moderate, and severe iris yellow
spot symptoms, respectively, and where ß0 ranged from -5.83
(for Copra) to +33.03 for (Ranchero).
In summary, IYSV had a similar significant negative impact on total
yield (t/A) and bulb size (%jumbo bulbs) for the 46 cultivars evaluated
under the conditions of this furrow-irrigated trial in a semi-arid
climate, but cultivars differed significantly in the total incidence
of plants with iris yellow spot symptoms and the incidence of plants
with mild, moderate, and severe symptoms. The results of this trial
may assist growers in selecting cultivars with some resistance to
iris yellow spot for planting in areas with a recent history of severe
outbreaks of this disease. However, empirical evidence indicates iris
yellow spot is significantly less severe n crops grown under overhead
(wheel-line, hand-line, or center pivot) irrigation compared to furrow
or drip-irrigation. The results may also be of assistance in breeding
onion cultivars for resistance to iris yellow spot.
