We’ve posted several in-depth articles on disease control and prevention here on our Seed Hopper blog before, and you can find them all archived in the “Plant Diseases” category in the left-hand navigation column. They include coverage of Late Blight, Downy Mildew and Disease Prevention in High Tunnel Production. Our “What’s Wrong with My Garden?” series goes over the basics of disease prevention and details control measures for many common diseases including: Powdery Mildew, Angular Leaf Spot, Downy Mildew, Bacterial Wilt, Early Blight, Late Blight, Septoria Leaf Spot, Scab, Black Rot, Fusarium Yellows, Club Root, Damping Off and Stewart’s Wilt. Also in our blog archive is a guide to diagnosing abiotic plant disorders, which may look like diseases but are in fact caused by environmental or cultural sources.

This post will go into some detail on diseases the following diseases - to jump to a certain section, click on the name of the disease you’re looking to learn about: Alternaria (Early Blight) · Black Rot · Botrytis (Gray Mold) · Cucumber Mosaic Virus · Rhizoctonia

Alternaria on carrot foliage.


Type: Fungal, various species

Crops: Potato, tomato, Brussels sprouts, broccoli, cabbage, cauliflower, kohlrabi, carrot, parsnip, parsley

How it spreads: Crop residues, weed hosts, pests, soil, seeds, machines/workers

Diagnosis: Symptoms include the appearance of dark brown or black lesions on foliage and/or stems (and eventually fruit), defoliation, yellowing and “bullseye” leaf spots.

Prevention: The best way to prevent alternaria, colloquially called “early blight,” is to start with disease-free soil and seeds. This fungal disease requires adequate moisture to activate spores, so adequate airflow and appropriate watering techniques are key for reducing risk. Good sanitation practices, crop rotation, and appropriate spacing and pruning of plants will help reduce the risk of spreading spores. Resistant cultivars (like Iron Lady F1) can also help fend off infections.

Mitigation: Once a crop has been infected it is very difficult to stem the flood of further infection. Some organically approved copper sulfate sprays can slow the spread of the disease. The most critical action to take once a field has been infected is to ensure that the disease does not have any favorable hosts to overwinter and appear again the following season. Good sanitation techniques and crop rotation will help prevent spores from over wintering.


Type: Bacterial (not to be confused with the fungal black rots that attack apples, grapes, citrus and sweet potatoes)

Crops: Broccoli, Brussels sprouts, cabbage, cauliflower, kale, kohlrabi, radish, rutabaga, turnip and mustard greens

How it spreads: Seeds, host weeds, machines/workers, occasionally pests

Diagnosis: Symptoms include yellowing, shedding of lower leaves, dark spots and blackened veins.

Prevention: Starting with disease-free seed is the best prevention method. Selecting resistant varieties (like Capture F1 cabbage) can also reduce the risk of infection. High Mowing tests all of our seed in the brassica family for black rot and black leg. No infected lots will ever be sold. Read more about our commitment to disease-free seed here. Other prevention techniques include hot water treatment of seeds (if purchasing from an uncertified source), crop rotation, and appropriate watering techniques to avoid unnecessary moisture among plants (bacteria can spread by splashing water).

Mitigation: Similar to Alternaria, once a crop has been infected it is very difficult to stem the flood of further infection. Some organically approved sprays can slow the spread of the disease. The most critical action to take once a field has been infected is to ensure that the disease does not have any favorable hosts to overwinter and appear again the following season. Good sanitation techniques and crop rotation will help prevent overwintering.

Botrytis on a lettuce.


Type: Fungal

Crops: Can affect a wide range of species (over 200 hosts have been reported) and is especially problematic in greenhouse tomato culture. Other important affected crops include cabbage, snap and lima beans, lettuce, muskmelon, peas, peppers and potatoes.

How it spreads: Crop residues, soil, seeds, host weeds

Diagnosis: Symptoms include lesions on stems and foliage (especially near a damaged area), and the appearance of a gray, velvety covering on flowers or fruit. White circular spots (“ghost spots”) can appear on fruit; plants will often abort infected fruit, leading to lots of fruit discard around infected plants. Pollen and flower parts that have fallen onto leaves are a common starting point for leaflet colonization.

Prevention: There is no known resistance to botrytis in tomatoes. Because the fungus spreads through moisture and splashing water, the best prevention techniques include controlling and carefully monitoring humidity and temperature. Adequate spacing, airflow, pruning and crop rotation can help reduce the risk of outbreaks.

Mitigation: There are limited options for organic controls once botrytis fungal spores have started to spread. The best mitigation is good prevention practices. If an area is infected, after the crop is removed for the season diligent sanitation techniques and crop rotation will help prevent an outbreak the following season.

Cucumber mosaic virus on watermelon.


Type: Viral

Crops: Spinach and lettuces

How it spreads: Pests (most commonly winged aphids) and infected weeds (i.e., chickweed, milkweed, purslane, dayflower, etc.). Can be seed-borne, though it is a very unstable virus and therefore not commonly transmitted by seed.

Diagnosis: Symptoms include veinal browning, stunted growth, yellowing, mottling of older leaves and malformation of younger leaves.

Prevention: Resistant varieties such as Winter Bloomsdale have proven effective for production success. Good sanitation practices and holistic management practices like crop rotation, cover cropping and aggressive weed management can help break pest and disease cycles and prevent initiation and spread.

Mitigation: Reducing primary inoculum is critical to delaying virus epidemics, but if an infection is identified some mineral oil sprays have been known to reduce spread. Spraying border areas with mineral oil is also beneficial. The technique used is sophisticated; consult with your local extension service before applying any sprays.

Rhizoctonia (bottom rot) on lettuce.


Type: Fungal

Crops: Potatoes and lettuce

How it spreads: Soil, plant debris, potato tubers

Diagnosis: Commonly called “black scurf” when seen on potatoes and “bottom rot” when seen on lettuce. Symptoms for potatoes include irregular brown and black hard masses on the surface of potato tubers; although the masses are superficial and do not cause damage to the quality of the tubers, even in storage, they can be considered unmarketable due to the aesthetic damage they cause. If Rhizoctonia is present at planting, the most severe damage occurs under the soil and often goes unnoticed: the fungus attacks underground sprouts before they emerge from the soil and can either prevent emergence or stunt growth. Symptoms for lettuce include brown, sunken lesions on the lettuce midribs that are in contact with the soil. As the fungus progresses, the bottom of the plant will continue to rot through to the interior leaves and can result in the collapse of the head.

Prevention: Starting with disease-free seed and utilizing a rigorous crop rotation can help prevent the presence of rhizoctonia in fields. Using resistant varieties (like Red Tide, New Red Fire, Optima and Pirat lettuces) can reduce the risk of infection. Because potato plants are more susceptible to rhizoctonia damage before emergence, getting potato plants to emerge quickly in the spring is key to minimizing damage. Greensprouting can help with this in areas that experience damp springs.

Mitigation: If you suspect rhizoctonia is present in a potato field, harvesting tubers as early as possible after skin set can reduce black scurf significantly. Never use infected tubers as seed potatoes for planting the next season, as it will increase the risk of damage to emerging potato sprouts.


Do you have advice on how to prevent or control these or other diseases? Share your insights in the comments below!