Purple blotch (Alternaria porri)
Purple blotch attacks onion, garlic, leek and other Allium crops. Initially, small white sunken spots develop on the leaves. These enlarge and under moist conditions, turn purple with a yellowish border and are often covered with a sooty deposit of spores. After 3-4 weeks the leaves turn yellow and collapse. Bulbs may also be attacked, mainly at the neck. This can be seen as a yellow to reddish watery rot.
A good timing of sowing or transplanting can minimise purple blotch attack by A. porri, depending on the local environmental conditions. The fungus requires rain or persistent dew for reproduction. It can grow through a wide temperature range of 6 to 33.8 °C. Optimum temperature of fungal growth is 25 °C.
- Varieties with waxy foliage are generally more resistant than those with glossy leaves.
- Increased ploughing between seasons may reduce the disease.
- Increased spacing between plants also may reduce disease development.
- Other good practices include seed treatment, rotations, removal of crop debris and planting in well-drained soil.
- Under conventional production system fungicides are used when the disease is severe. In organic production systems no direct measures are allowed.