(Alternaria porri) attacks onion, garlic, shallot and other Allium crops. Initially, small white sunken spots develop on the leaves. These enlarge, become zonate and under moist conditions, turn purple. These are also prominent on the inflorescence stalks. Infection can cause a semi-watery rot on necks of bulbs that turn yellow-red in color. Infected bulb tissues eventually become papery. This pathogen is widespread. The optimum temperature for disease development is 21 to 30° C. Therefore, it is most serious in hot, humid climates. The fungus is seed-borne, but the relevance of this phase in initiating disease outbreaks in hot climates is not well documented. Infected onion debris has been implicated as an infection source.
Host-plant resistance is yet to be exploited. Some cultivars appear to be less susceptible, e.g. Red Creole Taliana Red in Hungary is reported to be resistant. Cultural control methods include long rotations with unrelated crops and good drainage. Lowering the density of transplanted crops will reduce infection, as will the application of high rates of calcium superphosphate and potassium fertilizer. Nitrogen fertilizer at low and high rates will increase the prevalence of disease. Routine (weekly interval) field sprays with dithiocarbamate fungicides, particularly mancozeb and chlorothalonil have been reported to be effective.
Stemphylium Leaf Blight
(Stemphylium vesicarium) has been reported from Europe, Africa, North and South America, and Asia. Foliage losses of 80 to 90 percent have been reported. Disease symptoms are very similar to purple blotch. Lesions are light yellow to brown, watersoaked and progress from the tip to the base of leaves. The conidia have up to six transverse septa, besides several vertical septa. Wet and warm conditions favor the disease spread. Control measures are similar to purple blotch.
(Colletotrichum gloeosporioides) favors hot (24 to 29°C) and wet conditions. The disease overwinters in sets and soil, and spores are spread by wind, splashing water, and tools. The leaves become twisted due to infection.
(Peronospora destructor) also attacks young plants, appearing as white specks, usually confined to the oldest leaves of young plants. A white mold develops rapidly in cool damp weather and progresses down the sheath, and plants eventually fall over and dry up. The fungus overwinters in bulbs and sets and on plant debris. Spores are carried long distances by air currents. For control, young plants can be treated with mancozeb at weekly intervals until bulbing begins.
Botrytis Leaf Blight
Commonly termed blast, is caused by several Botrytis species. The disease first appears as white specks on leaves, expanding to cause a dieback from the leaf tips. Tops may be killed completely within a week, and entire fields may be affected. Frequently, blight follows previous damage from insects, disease, mechanical damage, or air pollution. Control is achieved through mancozeb sprays at approximately 7-day intervals.
Several bulb rots may occur either in the field or in storage. Basal rot, caused by Fusarium species, results in a breakdown of inner scales. Outwardly, the bulb may appear normal. It eventually becomes soft, however, and will develop a watery rot under moist conditions or a dry shriveled bulb in a dry environment. The disease is most severe in warm areas with poor soil drainage. Botrytis neck rot (shown) is an extension of the leaf blight disease and can become serious in storage.
Insect Pest Control Thrips
(Thrips tabaci) are minute insects that cut or “rasp” the epidermis of leaves or stems and suck the plant sap resulting in white blotches on leaves. Severe infestations result in leaf blasting and collapse. Bulbs become distorted and undersized. Infestations are more severe in dry seasons than in moist, and entire fields may be destroyed. The insect has many host plants. Adults and nymphs overwinter on plants or plant debris, or in weeds bordering the field. Most of the insects are female, which can reproduce without a male. Eggs are thrust into the leaves and will hatch in 5 to 10 days.Diazinon sprays at 7-10 day intervals are recommended to control thrips. Up to six applications may be necessary and good coverage is essential.
Onions are ready for harvest when the leaves collapse. For storage, onion tops should have broken over before harvest and the necks should collapse and dry. Storage bulb maturity can be accelerated by withholding irrigation water or by pruning the root system. Bulbs for storage may be harvested when 50 percent of more of the tops have broken over, but the bulbs must be cured and dried thoroughly before being placed in storage. Bulbs intended for immediate use can be undercut when 15 to 25 percent of the tops are down.
To harvest, first a knife or lifter is drawn under a bed or row, cutting roots and loosening the soil. Then the bulbs may be dug or allowed to cure further before digging. Under dry conditions, bulbs may be left to cure in the field, either in place or in windrows. To avoid damage from direct sunlight, however, onions normally are placed in field containers and moved to a dry shady location for subsequent curing.