Onion and garlic plants are also known as Allium sepa and Allium sativa. These plants are susceptible to certain fungal and bacterial diseases that affect the health of the plant. Growing healthy allium plants requires proper sanitation, avoiding plant wounds and proper use of pesticides.
Allium leaves can develop a disease known as Stemphylium leaf blight, which is caused by the fungus Stemphylium vesicarium. Symptoms associated with this disease include small, brown watery lesions that appear on the leaves of the plant. Blighted allium leaves result from these lesions growing into larger spots. According to the University of Arizona, Stemphylium vesicarium usually attacks dying tissue in plants, such as purple blotch, injured tissue and downy mildew lesions. Leaves are usually the only part of the plant affected by this disease. Wet, warm weather favours Stemphylium leaf blight.
Botrytis Leaf Spot
Botrytis leaf spot is a disease that commonly occurs on Allium sepa, but not on Allium sativa. This disease is caused by the fungus Botrytis cinera and is characterised by lesions on plant leaves surrounded by a halo of green colour. Onion plants develop this disease from spores in infected soil. The University of Arizona states that encouraging air movement and drying plant leaves prevents the development of Botrytis leaf spot. Fungicides often reduce the severity of this disease and lessen the damage to allium leaves.
Xanthomonas Leaf Blight
Xanthomonas leaf blight commonly occurs in the onion, or Allium sepa, plant. Warm temperatures and high periods of moisture encourage the development of this bacterial disease. Xanthomonas is particularly active during the months of July and August and very severe outbreaks often occur after long periods of rain. Bacteria enters the plant through wounds. Symptoms of this disease usually appear on middle-aged and older leaves. Colorado State University lists the symptoms of Xanthomonas leaf blight as lesions on plant leaves, with some being water-soaked. Streaks may run down the entire length of the plant and tip dieback often occurs.
Another symptom is a stunted plant with small bulbs and plants often die prematurely. The onion plant develops the most serious symptoms from Xanthomonas leaf blight but other plants such as the Allium sativum (garlic) and Allium porrum (leek) are also affected. Spraying with bactericides can prevent or reduce the symptoms associated with this disease.
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