Leaf Blight

Irregular brown or white spots on leaves often mean that leaf blight has taken hold.

Leaf blight on taro (Scot Nelson) Public Domain

There are many different types of leaf blight. Leaf blight can be caused by fungal spores or bacteria and most forms of leaf blight are species specific, which means different types of leaf blight attack different plants. If you grow onions or garlic, carrots, sorghum, melons or squash, you need to know about leaf blight.
 
Fungal leaf blights of onions and garlic
 
Purple lesions, yellow to brown spots, and yellow streaks on onion or garlic may mean purple blotch and Stemphlyium leaf blight have infected your plants. These two fungal leaf blight diseases are mostly late-season problems, so you can avoid them altogether by planting early season crops, if that's an option for your area. As a double whammy, these two diseases often start growing in downy mildew lesions. A similar leaf blight attacks sorghum.

Fungal leaf blight of squash and melons
 
Alternaria leaf blight is a fungal disease caused by Alternaria cucumerina that attacks members of the squash family. Melons are the most susceptible, but squash, pumpkin and cucumber may also become infected. Brown smudges first appear on older leaves, near the base, or crown, of the plant. If you look closer, you will see that the damaged areas often have a yellow halo. These leaf spots may also develop concentric, target-shaped rings. Ultimately, leaves wither, curl upward, and die. Alternaria leaf blight doesn't harm the fruit, but it can interfere with photosynthesis enough to reduce crop size and can cause sunburn damage.

Bacterial leaf blight of carrots

Bacterial leaf blight is caused by Xanthomonas campestris pv. carotae and attacks carrots. Brown spots are first seen along leaf edges, or margins, and may look watersoaked. As the disease spreads, you may be able to see dark brown streaks on the petioles, or leaf stems. Flowers may also become affected. An amber-colored ooze is usually present, as well.

Bacterial leaf blight is a seed borne disease, so be sure to get your seeds from reputable suppliers. This disease is only active when moisture is present. Optimal temperatures are between 77° and 86°F. This disease does not occur when temperatures are below 65°F.

Leaf blight management
 
Preventing disease is nearly always easier than curing it, and leaf blights are no exception. You can prevent leaf blights in your garden with these tips:

• avoid overhead watering
• use a 2- or 3-year crop rotation
• do not work around plants while leaves are wet

 
If leaf blight appears in your landscape, toss infected plants in the trash at the end of the growing season and be sure to sanitize your garden tools between cuts. You can use common bathroom cleaners to do this.