A Scoop on Tomato Blight: A Gardener's Worst Nightmare
Identifying, Preventing, and Treating Common Tomato Blights: A Comprehensive Guide
Every green-thumbed hobbyist and professional gardener knows the nightmare that is tomato blight. This catch-all term for a gaggle of fungal infections can wreak havoc on tomato plants, causing reduced fruit production, damaged or dropped fruit, plant collapse, and even plant demise. What's worse, these pesky fungi strike at different times and in various ways, making identification critical for effective control measures.
Tomato blight ultimately results from moist conditions and moderate temperatures. This diabolical duo encourages the growth of these fungal diseases, which often survive in plant debris, weed hosts, or infected seeds/transplants. They then spread through rain, overhead irrigation, movement of infected soil or plant debris, and working in infected plant zones. So, let's get our sleeves rolled up and learn about the main types of tomato blight and tactics to stop these malicious microorganisms in their tracks.
The main types of tomato blight: The Insidious Enemies
Early Blight
Kicking off our foes list is early blight, triggered by the fungal pathogens Alternaria solani and Alternaria tomatophila. This treacherous tomato blight affects stems, leaves, and fruit. Infections occur when lower leaves come into contact with contaminated soil or when water droplets splash upwards from infected soil onto the leaves.
Early blight symptoms usually appear after the first fruits have begun to bear. Characterized by dark brown to black lesions, these almond-shaped blemishes first appear on larger, older leaves. As the disease progresses, the lesions grow and develop a concentric ring that resembles a shooting target or a bull's eye. The plant eventually becomes defoliated, and the fruit develops large, bruised spots and begins to fall. Fruit can be infected at any stage, with as much as 30-50% of immature fruit potentially lost. To minimize loss, harvest any ripe or semi-ripe fruit as soon as you spot symptoms on the leaves.
Early blight has no cure, but taking action to slow its severity can help minimize crop losses. Remove infected leaves (up to a third of the plant's foliage), dispose of them by burning them, and clean your hands and tools after handling infected plants. Water plants from below to keep the leaves dry. For prevention, burn the infected plants after harvesting, grow early blight-resistant varieties, and practice crop rotation.
Late Blight
Arguably the most destructive and least common type of tomato blight is late blight, caused by the fungus Phytophthora infestans. This insidious invader can also attack other nightshade family members, such as potatoes. Unlike early blight, late blight prefers cooler temperatures combined with moist conditions.
Early signs of late blight manifest as dark green to brown lesions covered with a white fungal growth on leaves and stems. These nickel-sized lesions often appear water-soaked. As the disease progresses, the lesions grow larger, eventually girdling stems, causing the plant to wilt. If the lesions have a yellow border and occur on the lower portion of the plant, they are possibly due to early blight or septoria leaf spot.
Late blight also infects tomato fruit, causing green fruit to turn brown and develop water-soaked spots. Mature fruits rot rapidly. Pruning leaves may slow its spread, but the best course of action is to quickly harvest and use the fruit, then dig up and burn infected plants. Disinfect any equipment that came into contact with infected plants. Avoid growing tomatoes or potatoes in the same location for four years to prevent re-infection.
Septoria Leaf Spot
The most common tomato blight, even though it's the least harmful, is Septoria leaf spot. Caused by Septoria lycopersici, this blight usually appears in late July after a period of wet and humid weather, revealing itself as small black or brown marks on the lower leaves. These cankers stress the plant, causing it to shed leaves.
Though fruit is usually unaffected, excessive leaf loss can affect yield and increase the risk of sunscald. Removing infected foliage can slow the progression of the disease. Additionally, watering plants only at the base level can help. While a fungicide can be used as a last resort to protect new growth, it won't cure infected foliage. Apply fungicide every 7 to 10 days throughout the growing season once the disease is detected.
Southern Blight
Lastly, Southern blight of tomatoes is caused by the fungus Athelia rolfsii (Sclerotium rolfsii). Prevalent in warmer climates, this aggressive blight thrives in high temperatures (77-95°F/25-35°C) and moist conditions. It spreads through moving infected plant material or soil.
The first symptoms appear on the lower stems at soil level as brown to black lesions that rapidly girdle the stem, causing the plant to wilt. These lesions are usually accompanied by white mycelium, which turns into sclerotia after a few days. Fruit may also become infected with yellowish, sunken lesions that eventually rot and release a foul odor.
Remove infected plants as soon as symptoms appear, burn them, and sanitize all equipment. Fungicides can help control the spread of the disease, but they are more effective when used preventatively in problem zones.
Defending your tomato patch: The battle plan
An active tomato blight infestation can't be cured - only managed. Prevention is key, so be sure to:
- Grow disease-resistant tomato varieties;
- Remove damaged lower leaves regularly since that's where most fungal attacks start;
- Rotate crops for 3-4 years with a successive non-nightshade plant;
- Keep the area clear of plant debris and potential host weeds;
- Dispose of all plant debris at the end of the growing season so spores have nowhere to overwinter;
- Maintain a healthy, well-fertilized soil and irrigated garden landscape;
- Adopt drip lines, soaker hoses, or hand watering at the base of plants instead of overhead watering, which encourages moisture on leaves; and
- If possible, avoid the garden while foliage is wet from dew or rain.
Changing planting dates to avoid weather conditions that encourage the transmission of tomato blight diseases can also help prevent infections. When in doubt, throw away questionable fruit.
In summary, tomato blight is a persistent threat to tomato crops. By being vigilant, practicing good crop management, and growing disease-resistant varieties, gardeners can stand a fighting chance against these relentless foes.
Home-and-garden enthusiasts and avid gardeners can protect their tomato plants from blight by adopting appropriate gardening methods. By growing blight-resistant tomato varieties and maintaining a clean, well-tended home-and-garden, gardeners can minimize the risk of tomato blight infections. Regularly removing lower leaves, practicing good crop rotation, keeping the garden free of debris, and maintaining a healthy, well-fertilized soil are essential preventative measures. Additionally, adopting drip lines, soaker hoses, or hand watering at the base of plants instead of overhead watering can help prevent moisture buildup on leaves, thereby reducing the risk of blight.
