Called Alternaria spp. by science, this fungal infection is the bane of tomato and cole crop growers everywhere. There are three species of Alternaria blight:
Alternaria leaf spot for tomato – Alternaria stem canker (A. alternata)
Alternaria leaf spot for cole crops – Alternaria blight (A. brassica or A. brassicicola)
Leaf blight of carrot – Black rot of carrot (A. dauci or A. radicina)
Alternaria potetially affects all varieties of tomato, carrots, crucifers, beans, and less often other herbaceous plants. It attacks at all growth stages, though it is most common during the flowering and fruiting stage.
Symptoms of Alternaria
Alternaria fungus is seed borne, so it is transferred through infected seeds to the affected plant and spread to nearby plants as well. Most commercial seed houses screen for infection before distribution, but do not always catch the problem.
Plants that are infected will show initially as damping-off and stunted seedlings, but because infection develops so slowly, a sick plant may not show many signs until well into growth. In fact, it’s rarely caught before transplantation and fruiting begins – in this way often infecting all nearby plants in the process.
Most often, gardeners will notice the blight when it begins appearing on leaves as spots that range in size from tiny to the width of your thumb. They are usually yellow or brown and once they appear, they spread rapidly in concentric rings of darkening color. Spots usually transfer or begin appearing on fruit and seed pods at about the time they appear on leaves.
Alternaria has a long-term life cycle compared to most fungi. Its life begins as a spore embedded onto the coating of a seed, often waiting through the winter and spring just as the seed will. It then spreads slowly, attaching to other plants the infected host might come in contact with and even having the ability to “blow” spore across short distances to other plants. It can also live on some soil materials, though conditions must be perfect.
Once the fungus has begun to show on the plant, it’s likely that it is nearing its spore stage. It will infect the fruit of the plant and then often kill the host at about the time the fruit drops to begin the cycle anew.
How This Plant Disease Impacts the Plant
On most plants, it will stunt production and infected fruits are not considered edible. It’s dangerous to feed infected fruits to animals as this is suspected to cause gastro-intestinal problems (usually diarrhea) in some livestock. Tomatoes will show slow production and smaller than average fruit while carrots will wilt in the ground without growing to potential. Beans and coles will be stunted and cabbage heads may never develop properly. In all plants, a rush to flower and seed (“bolting”) is almost always the first symptom noticed.
How to Prevent Alternaria
Prevention is about seed selection and the segregation and plants suspected of being infected. Quarantined plants (usually enclosed in plastic) may be watched for signs of infection. Most growers will just remove and destroy suspect plants as a matter of course. Infected plants should never be composted, fed to livestock, or otherwise consumed.
Wider spacing, clear weeding, and plant staking to allow air flow are more ways of preventing blight.
Once it’s there, Alternaria cannot be removed easily. Most gardeners remove and destroy the whole plant. Fungal treatments can help, but are not a guarantee and with the risk of infecting all other plants in the area, most growers take the sure route of destruction of removing the plants.
If your plants are isolated or you’re unwilling to pull and destroy, try first an organic fungicide. A good one to try is GreenCure. It’s an economical foliar organic fungicide that can be used for treatment and prevention on flowers, trees, turf, houseplants, and gardens. To use GreenCure as a prevention method to Alternaria Blight, simply add 1 tablespoon per gallon of water every one to two weeks.