Crop Profile for Onions in California - Regional IPM Centers

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CHEMICAL CONTROL ● Dicloran – see white rot. Basal rot (Fusarium oxysporum f.sp. cepae). Fungi that can survive indefinitely in the soil cause basal rot. Symptoms of basal rot are a progressive yellowing and dieback from the leaf tip, dark brown roots and occasionally, a white fungal growth at the base of the bulb. Basal rot is often associated with pink root, maggots, or other insect injury and is more prevalent in transplants. Optimum conditions for infection are moist soils with temperatures ranging from 79 to 82° F. Basal rot may progress after harvest if the bulbs are stored in moist conditions. (12) CULTURAL CONTROL- Onions should not be planted in fields with a history of basal rot. Basal rot infection can be minimized by a three to four year crop rotation (crops other than onions, garlic or leeks), proper curing prior to storage and maintenance of cool storage temperatures. (12) CHEMICAL CONTROL ● Methyl bromide – see white rot. Pinkroot (Phoma terrestris). A fungus that can survive in soil indefinitely causes pinkroot. Drainage water or dirty equipment can easily spread the fungus. Optimum conditions for pinkroot are temperatures ranging from 75° F to 85° F with high relative humidity. (12) Pinkroot causes roots to turn pink, darken and eventually die. Roots produced thereafter become diseased and die. This continues throughout the growing season, and results in stunted bulbs. (12) CULTURAL CONTROL- Several varieties of onions such as Durango, Yula, Yellow Globe Danvers, and others are resistant to certain strains of pinkroot. Crops should be on a five-year or more long-term rotation out of onions and not be planted following a cereal crop. Proper fertilization, irrigation, cultivation, and insect control will minimize infection. Solarization of the soil is also used in the San Joaquin Valley to prevent pinkroot. (12) CHEMICAL CONTROL
● Chloropicrin was applied in seven treatments using 4,069 lbs. a.i. in 1997. Applications by injection are made two weeks prior to planting at rates of 200 to 400 lbs. per acre. (3,5,11,12,13,14) ● Dichloropropene is a preplant treatment that is applied by ground or by injection at rates of 103 to 285 lbs. a.i. per acre. This pesticide can also be mixed with chloropicrin. It was applied in four treatments using 10,801 lbs. a.i. in 1997. (3,5) ● Metam sodium - see bulb mites. Bacterial soft rots (Erwinia carotovora carotovora, E. chrysantherni, Pseudomonas gladioli, and Entererobacter cloacae). Bacterial soft rots are caused by soil borne organisms that can be spread through irrigation or rainwater. Symptoms appear at harvest or later. They can be characterized by softening and water soaking of one or more of the inner fleshy scales of the bulb, yellowing of infected tissue and a softening of the bulb neck. (12) CULTURAL CONTROL- Once onions start to bulb, soft rot infections can be minimized by using furrow irrigation rather than sprinkler irrigation. Harvest should be made only after onions have fully matured and the onions should be cured prior to storage. (12) CHEMICAL CONTROL- No chemical controls are available for soft rots. (12) Black mold (Aspergillus niger). Black mold is caused by a fungus that is found in desert areas of California. The fungus survives in the soil on decaying organic matter and infects damaged onions by entering via wounds. Symptoms of black mold are moldy growths appearing on the top or sides of bulbs, water soaked scales, and black masses of spores that become visible between outer scales. (12) CULTURAL CONTROL- There is no direct control of black mold, however infection can be reduced if bruises and other damage to bulbs is minimized during handling, and if keeping storage and transportation temperatures are maintained between 33° F and 55° F. (12) CHEMICAL CONTROL- No chemical controls are available for black mold. (12)
Page 1 and 2: Prepared: October, 1999 Crop Profil
Page 3 and 4: tape. Seeds are planted half an inc
Page 5 and 6: damage. Under dry, hot conditions a
Page 7 and 8: ● Pyrethrins were applied in 43 o
Page 9 and 10: ● Zeta-cypermethrin is an onion m
Page 11 and 12: in 1997. Rates typically used are 0
Page 13 and 14: Diseases Diseases can pose a seriou
Page 15 and 16: Purple blotch and Stemphylium leaf
Page 17: A fungus causes botrytis leafspot.
Page 21 and 22: CULTURAL CONTROL- Avoid planting in
Page 23 and 24: weeds can occur during one crop cyc
Page 25 and 26: depleted will no longer be availabl
Page 27 and 28: Mike Davis Plant Pathologist Dept.
Page 29 and 30: 12. University of California, Pest
Page 31 and 32: lettuce, prickly C C N C P N C N N
Page 33: CLE = clethodim PAR = paraquat FLU

Crop Profile for Onions in California - Regional IPM Centers

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