2002 Western Colorado Onion Variety Trials

Pink Root Study

Dr. Curtis E. Swift, Area Extension Agent (Horticulture), and A. Wayne Cooley, Extension Agent (Soil and Crop Science) (deceased)

Purpose:

Pink root, caused by Phoma terrestris E.M. Hans. [syn. Pyrenochaeta terrestris (E.M. Hans.) Gorenz, J.C. Walker & R.H. Larson], is one of the most devastating diseases of onions grown in warm climates (Schwartz and Mohan, 1995, pg. 12) and is a wide-spread problem of onions in Colorado (Everts, 1984, pg. 3.).  In this study 27 varieties were examined to determine their susceptibility/resistance to this disease.

Materials and Methods:

Two commercial onion fields farmed by Jerry Hines (Delta County) and Duane Homewood (Montrose County) were used in this study.  These fields varied in their soil types (silty clay loam vs. a soil of a sandy consistency respectively), and history of P. terrestris contamination.  The Hine’s field has a history of a higher percentage of pink root than the Homewood field.

Seed dealers participating in this trial are listed in Table 1.   They specified where each variety would be planted (Delta or Montrose site).  Varieties examined but not listed in Table 1 were included at the cooperator's request. The growing season at the Montrose site was 95 - 110 days; the growing season at the Delta county field varies from 110 - 120 days.

Table 1: Varieties included in this trial

Variety		Seed Company	Length of growing Season	Type of Onion
Eagle T‑406		American Takii	117	Spanish Storage
Frontier T‑400		American Takii	97	Export
T‑441		American Takii	107	Hard Globe
T‑439		American Takii	107	Full Spanish
T‑418		American Takii	97	Export
Husky X302		Palmer	90	Hard Globe
DPS 1169		Palmer	92	Hard Globe
DPSX1170		Palmer	112	Spanish storage x hard globe
Kodiak X400		Palmer	112	Spanish storage x hard globe
DPSX1171		Palmer	116	Full Spanish
DPSX1172		Palmer	116	Full Spanish
Expression		Bejo	98	Intermediate
BGS‑142		Bejo	105	Export
Prince		Bejo	105	Export
Delgado		Bejo	125	Spanish Storage
Legend		Bejo	125	Spanish Storage
Daytona		Bejo	120	Spanish Storage
Seahawk EX‑15232		Seminis	115	Spanish Storage
Kingfisher EX‑15120		Seminis	117	Spanish Storage
Super Chief EX‑77031		Seminis	115	Full Spanish
Tioga		Seminis	115	Spanish Storage
Varsity		Seedworks	105	Spanish Storage
Raptor		Seedworks	118	Full Spanish
Xph95345		Crookham	115	Spanish Storage

A randomized complete block design was utilized with each variety replicated four times.  Plots were 35 feet long; two lines of the same variety of onion were planted on each bed.  Beds were 30 and 34 inch on center at the Homewood and Hines fields, respectively.  All onions in a ten-foot lineal space were hand-dug from each line of onions in each plot and graded as to the percentage of roots infected by the pink root organism (as determined by color). The data for each 10-foot section of row were recorded separately. The average percentage of infection for the resulting twenty feet of row was analyzed using MSTAT-C, the Michigan State MicroProcessor Statistical Program. 

Past experience indicated pink root infections occur in pockets in the field/row.  Harvesting all bulbs in a continuous section of row was felt to be a more realistic test of the amount of infection present than harvesting bulbs at random throughout the plot.

The statistical difference of susceptibility to P. terrestris is indicated in Table 2 by the letter(s) that follow the percentage of infection.  Percentages followed by the same letter are statistically identical in regard to incidence of pink root.  In this trial, "a" following the percentage of pink root indicates the higher level of infection; "h" in Field #1, and "e" in field #2 following a percentage indicates the lowest level of infection.

Table 2: Percentage of Pink Root in Field #1 and Field #2

Field #1:  Hines, Delta County		Field #2: Homewood, Montrose County
Variety	Percentage of Pink Root	Variety	Percentage of Pink Root
Varsity	41.76 a	Varsity	10.29   a
Delgado	41.62 a	Prince	9.19   ab
Eagle (T-406)	40.60 a	T-418	8.64   abc
Legend	25.15  b	T-439	8.51   abc
Seahawk (EX-15232)	23.81  bc	Frontier (T-400)	7.52     bcd
Daytona	22.83  bc	DPS 1169	6.46       cde
T-441	21.34  bcd	Teton	5.98         de
Kodiak	20.06  bcde	Expression	5.70         de
DPSX 1172	19.65  bcdef	Husky	5.52         de
DPSX 1171	17.32  bcdefg	DPSX1170	4.87           e
Raptor	16.09  bcdefg	BGS-142	4.69           e
Kingfisher (EX-15120)	14.32    cdefgh
Xph95345	12.52      defgh
Ranchero	10.53        efgh
SuperChief (EX-77031)	9.72          fgh
Granero	7.59             h
Tioga	5.32             h
LSD	10.7   0.01	LSD	2.398  0.10

Results:

Table 2 shows the percent infection of each variety examined ranged from a high of 41.76 to a low of 4.69 in the two fields, with the Delta county field having the higher infection rates.  The highest level of infection in the Montrose county field was 10.29%.

Discussion:

Data collected in 2002 indicate varietal differences in susceptibility to the pink root organism.  Varieties that appear more resistant and having qualities of size and harvest date preferred by the grower should be selected for planting in fields known to have a history of P. terrestris contamination. Varsity, included in both trials, had the highest percentage of infection in both trials.

Acknowledgments:

Without the help of Jerry Hines and Duane Homewood and their staff these plots would not have been possible.  They were responsible for planting, cultivation, irrigation and many other tasks. Tri River Area Master Gardeners devoted considerable time assisting in planting, harvesting and collecting data.  American Takii, Inc., D. Palmer Seed CO., Inc., Bejo Seeds, Inc., Seminis Vegetable Seeds, SeedWorks, and Crookham Company provided seed and financial support for these trials. Financial support and encouragement was also provided by the Colorado Onion Association.

References:

Schwartz, H.F., and Mohan, S.K. 1995. Compendium of Onion and Garlic Diseases. APS Press, St. Paul, MN.

Everts, K.L. 1984. Masters Thesis: Epidemiology and Management of Fusarium Basal Rot and Pink Root Diseases of Onions. Colorado State University, Fort Collins, CO