Diseases can occur at any stage during the course of plant growth. The rapid, accurate diagnosis of the cause of a disease, along with the implementation of a rapid treatment, is essential to ensure the protection of the crop.

Certain infectious diseases caused by living, microscopic organisms have the potential to rapidly ruin a crop. However, for any particular vegetable, these diseases are not that numerous and, so, it would not be difficult for a grower to become familiar with them and take proper preventative action. Diseases caused by nonliving things (i.e. not infectious) can be much more difficult to diagnose. Usually, it is easier to rule out an infectious agent as the cause of a disease before investigating possible nonliving (abiotic) causes.

This stresses the need for the farmers to become familiar with the more common infectious diseases that can occur on the crop. This post provides an overview of the science of diagnosis and treatment of vegetable diseases.

Why Is There Disease?

Disease is the outcome of an interaction between the host, the disease agent, and their environment. If the cause of infectious disease, the pathogen, is next to the host, nothing will happen unless environmental factors are favorable for its infection and development within the plant. With foliar pathogens, there is usually a minimal period of leaf wetness required to stimulate spore germination and infection. For some soilborne pathogens, infection occurs in combination with high soil moisture and certain critical soil temperatures.

Knowledge of conducive environmental factors for the more important vegetable diseases presents an opportunity for more effective management: the disease can prevented by altering some of the environmental factors, or, when such factors cannot be altered, steps can be taken to minimize the impact (e.g. fungicides could be applied in advance of a period of sustained rain which would favor foliar diseases.)

Diagnosing Diseases of Vegetables

Not all diseases are caused by pathogenic organisms. Determining whether a disease is caused by a pathogen, or has nonliving (abiotic) causes requires not only the examination of individual plants, but also, noting the pattern of symptom occurrence in a field. Examine individual plants for unusual symptoms, such as leaf spots, wilts, stunting, fruit rots, misshapen leaves, cankers and stem blight. Roots should be examined for galls, root rot and necrosis (dead areas). Fields should be observed to determine if the problem is widespread and whether different plants species in and around the field are affected, which could indicate an abiotic cause.

Symptoms with a nutritional or physiological cause have a more widespread occurrence within a field than infectious diseases. Initially, most disease causing pathogens will be isolated in areas and spread outward from those areas. Also, weeds or nonrelated crops are not typically affected. Soilborne pathogens are even more restricted within a field than foliar pathogens.

Diagnosis of Diseases Caused by Different Types of Pathogens

Fungi

Fungi are multicellular microscopic organisms that can grow to their food, usually in the form of filamentous strands. Their growth pattern is radial, so on surfaces such as plant leaves, the effects of their growth may be seen as circular spots. However, fungal infections of other plant parts, such as roots, may produce no visible structures. Some symptoms can indicate these infections. For example, browning of the water-conducting tissues of the stem, in combination with wilt, can indicate infection by the Fusarium wilt fungus.

Other disease symptoms, such as blight (a general death of tissue), which can have a variety of causes, may require laboratory testing to confirm fungi as a cause. Fungi can produce specialized structures, such as spores, which are used for reproduction, dissemination through space and time, and survival. Sclerotia are structures that function in the long term survival of many soilborne pathogens. The southern blight fungus which infects many vegetables forms sclerotia resembling mustard seeds.

Most fungi that infect leaves require free moisture to initiate infection, with the exception of powdery mildew fungi, which need only high humidity to initiate infection.

Bacteria

Bacteria are single celled microscopic organisms, which survive by becoming dormant. The most notable exception to this is the pathogen that causes common scab on potato: this is a filamentous, multicelled bacterium that produces spores. They can be transported by wind driven rain, by insects, or the movement of infected plant parts, including seed. Bacteria that infect leaves may cause circular spots, but irregular shaped lesions that don’t extend beyond veins are more characteristic for bacterial infections.

They can also cause soft rots of vegetative parts that are usually characterized by a foul smell. Other bacterial diseases that produce symptoms such as wilt require laboratory analysis for diagnosis.

Mycoplasmas are bacteria-like, only structurally simpler and smaller than bacteria. They are transmitted by leafhoppers. The most notable disease of vegetables caused by a mycloplasma is aster yellows, which affects carrots, celery and related plants. The symptoms of aster yellows are distinctive: leaves have a bronzed appearance and flowers are abnormal (leaf-like tissue grows from them, instead of petals.)

Characteristics to evaluate when Diagnosing Diseases

General Symptoms	Specific Symptoms	Possible Causes
Distribution in the Field	Symptoms observed over a wide area on several crops	Soil Problem Low Fertility Insect Injury Physiological Problem
	Symptoms observed over a wide area on a single crop	Soil Problem Low Fertility Insect Injury Virus Foliage Pathogen (advanced stage of epidemic)
	Symptoms observed scattered over a field on a single crop	Soilborne Root Rot Fungi Soilborne Wilt Organisms Nematodes Virus Foliar Pathogens Soil Problem Insect Injury
Foliage symptoms	General yellowing	Wet Soil Low Fertility Root Rot Pathogens Nematodes
	Necrotic spots on leaves	Spots Generally Round (Fungal leafspot) Spots Generally Angular (Bacterial leafspot)
	White powdery substance on leaf surface	Powdery Mildew
	Light yellow spot on upper leaf surface downy growth on lower surface	Downy Mildew
	Ruptured areas on lower leaf surface	White in center of ruptured area (White Rust) Reddish brown to orange in center of ruptured area (Rust)
	Light and dark green areas on a leaf	Appears in a random pattern in the field (Virus) Appears in rows (Nutrient Deficiency)
	Leaves distorted	Virus Herbicide
	Leaves with holes or chewed areas	Insect Injury
	Leaves yellow, wilt and die	Root decayed (Root Rot) Brown ring in vascular portion of root (Fusarium Wilt) Roots cut or damaged by feeding (Insects; Gophers or Moles) Damage in low areas of field (Root Rot; Poor Drainage)
Root Symptoms	Roots decayed	Root Rot Poor Drainage
	Roots with discoloration beneath outer layer	Fusarium Wilt
	Swellings on roots and stem	Large swelling (Crown Gall Bacterium) Small swellings which appear in a random patter on roots (Root Knot Nematode) Small swelling at root tips (Dagger Nematode)
	Root tips dead	Isolated areas (Nematodes; Fertilizer Burn) Affected area is in a pattern or over the general field (Fertilizer Burn)
	Roots dead with white fungal strands around the stem at the soil line	Southern Blight
Fruit Symptoms	Fruit decay scattered over fruit surface	Watery soft decay, foul odor (Bacterial Soft Rot) Firm to watery soft rot (Fungal Decay)
	Hard black decay at blossom end	Blossom end rot (Nutrient and Water Problem)
	Fruit distorted	Virus Insects
	Faint rings visible on fruit	Virus
	Light colored blotchy appearance	Insects
	Dark raised areas on fruit	Bacterial Leafspot

Viruses

Viruses are submicroscopic entities that can only replicate inside living plant cells. They require agents such as insects to transmit them to plants. A typical virus symptom is a mosaic pattern on leaves, but viruses may cause other symptoms, such as necrotic lesions and stunting, which can have other causes. Insect vectors of viruses include by aphids, leafhoppers, white flies and thrips. Viruses often have wide host ranges, including weeds that are not botanically related to the crop, and symptoms are not always produced in these plants.

Although some viruses can tentatively be identified by symptoms produced on plants (particularly mosaic or ringspots), accurate diagnosis requires laboratory testing.

Nematodes

Nematodes are microscopic roundworms that feed on the roots of plants. The root knot nematode produces root galls and deformed roots on a wide range of crops. Heavy infestation can lead to wilting and death of plants. Nematodes can also cause stubbiness, necrosis and stunting of roots, but these symptoms are not distinctive. Soil or root analysis is required to confirm diagnosis.

Environmental Factors Affecting Plant Pathogens

Temperature

Soil temperatures that are suboptimal for seed germination and seedling growth can favor the development of damping off pathogens, such as Pythium and Rhizoctonia. Often, the impact of these pathogens can be reduced by delaying planting, until the soil temperature increases. Foliar pathogens have different optimal temperatures for development. Symptoms of bacterial diseases on leaves are produced at high temperatures, at which a fungal disease like downy mildew might be halted. These temperature requirements are known for many diseases and with a disease like late blight of potato, can be used to accurately apply fungicides when they are needed.

Moisture

Moisture is the driving force for foliar diseases. Free moisture on the plant surface is necessary for growth and infection of many bacteria and fungi. There is usually a minimum, critical time requirement. Moisture can also infect spore production by fungi. This characteristic is the basis of a predictive system for fungicide choice for management of purple blotch of onion.

Saturated soil moisture conditions can create an environment conducive for root-rotting pathogens such as Pythium or other fungi that cause fruit rot.

Soil pH

Soil pH affects the activity of some soilborne pathogens. Diseases caused by the cotton root rot fungus tend to occur in alkaline soils, while Fusarium wilt diseases are more prevalent in acidic soils. The growth of the common scab pathogen of potato is reduced in acidic soils. However, it is generally not feasible to alter soil pH to manage disease.

Disease Control Methods

Cultivar Selection

In some cases, vegetable cultivars may be resistant or tolerant to important diseases. Resistant cultivars have no or low severity of disease, while tolerant cultivars may become diseased without incurring yield loss. The use of these cultivars does not necessarily eliminate the need for other disease management approaches, such as the use of fungicides.

Disease-Free Planting Material

Several pathogens can be spread by contaminated seed, for example, the bacteria causing leaf spot of pepper and tomato, black rot of crucifiers and fruit blotch of watermelon; and fungi causing gummy stem blight and anthracnose on watermelon. Some seed companies indicate whether seed lots have been tested for these pathogens. However, low levels of seed contamination could increase if transplants are produced from the seed. Infected transplants may have slight or no symptoms. The use of a soilless mix and good sanitation practices during the production of the transplants will greatly reduce the risk of infection by soilborne pathogens.

The risk of introducing a foliar epidemic, such as gummy stem blight, can be minimized by inspecting transplants and making preventative applications of fungicides or bactericides (copper-based) early in the season.

Rotation

Many pathogens which cause diseases survive in the soil, on crop residue. Rotation with nonsusceptible crops, or fallowing, can reduce the pathogen population. Rotation is not a practical alternative for some soilborne fungi, such as Fusarium wilt fungi and the southern blight fungus, because their survival structures persist for years in soil. White rust, an important fungus disease of spinach, mustard, turnip and swiss chard, forms survival structures in the leaves of diseased plants that require 2 to 3 years to die out.

Burial of crop residue can augment the beneficial effects of crop rotation. Many foliar pathogens can survive in crop residue, but only for one or two years. Burial of crop residue decreases pathogen survival and physically removes their access to the new crop. Shredding of crop residue can speed up the decomposition of certain pathogens. For example, the bacterium that causes black rot of cabbage can survive in plant residue for nearly a year, but if the residue is shredded, the survival time decreases to less than 2 months.

A pathogen that attacks one member of a plant family frequently will infect other members of that family or group. Table VII-2 lists vegetables which are susceptible to similar diseases. A standard rotation program should use only one member of each group on a site in a 2 to 3 year period. An exception to this recommendation is the Fusarium wilt fungus. There are variants of this fungus that infect one crop only. Thus, the Fusarium wilt fungus that infects watermelon will not infect cantaloupe or other cucurbits and these crops can be grown in soils where Fusarium wilt of watermelon was a problem.

Rotation is important in the control of root knot nematodes. If susceptible crops are grown repeatedly on nematode infested land, production levels decrease until susceptible crops are no longer economical. Onions, shallots, garlic, leek or sweet corn should be grown on sites where root knot nematodes are a known problem. Fallowing can also reduce populations, but the fallow land must be kept weed free, since many weeds are also hosts for this nematode.

Common Diseases of Specific Vegetables and their Control

Crop	Disease	Control
Beans, Snap	Southern Blight	Apply preplant soil fungicide Deep burial of crop residue
	Damping Off	Planting at soil temperature > 60°F Plant on raised beds Plant treated seed Apply preplant soil fungicide
	Bean Mosaic (virus)	Virus-free seed Resistant cultivars
	Bacterial Blight	Pathogen-free seed Apply copper fungicides Two year crop rotation
	Cercospora and other Fungal Leafspots	Apply fungicides as needed Two year crop rotation
	White Mold	Apply fungicides as needed Wide row spacing and orientation to favor drying of the canopy
	Rust	Apply fungicides as needed Crop rotation Resistant cultivars
Cabbage, Cauliflower, Broccoli, Kale	Black Rot	Plant hot water treated seed Plant resistant varieties Plant on raised bed to avoid flooding Two year crop rotation
	Downy Mildew	Plant resistant varieties Apply fungicides as needed
	Alternaria Leafspot	Apply fungicides as needed
Cucurbits: Watermelon, Pumpkin, Cantaloupe, Squash, Cucumber, Cushaw	Fusarium Wilt	Crop rotation (5+ years) Resistant cultivars
	Powdery Mildew	Resistant cultivars Apply fungicides as needed
	Downy Mildew	Resistant cultivars Apply fungicides on a preventative basis
	Anthracnose (watermelon, cucumber, cantaloupe)	Resistant cultivars Crop rotation Apply fungicides on a preventative basis
	Alternaria Leafspot, Cercospora Leafspot	Apply fungicides as needed Two year crop rotation
	Angular Leafspot	Pathogen-free seed Apply copper fungicides Avoid overhead irrigation
	Bacterial Wilt (cucumber)	Control beetle vector Resistant cultivars
	Fruit Blotch (watermelon)	Pathogen-free seed Copper fungicides Two year crop rotation Avoid overhead irrigation
	Gummy Stem Blight	Two year crop rotation Apply fungicides on a preventative basis
	Choanephora Wet Rot	Frequent sprays of blossoms with copper fungicides Keep fruit cool and dry after harvest
	Virus Diseases	Resistant cultivars
Carrot	Bacterial Soft Rot	Minimize injury during harvesting, grading and packing. If carrots are washed after harvesting, they can be dipped in a 1:500 solution of sodium hypochlorite (5.25%) Store at a temperature just above 32°F.
	Crown Rot	Crop rotation (4 to 5 years)
	Cercospora or Alternaria Leaf Blight	Apply fungicides as needed
	Aster Yellows	Control leafhoppers Control weeds in and around fields
Corn, Sweet & Pop	Northern Corn Leaf Blight	Burial of crop residue Apply fungicides as needed
	Rusts (Common, Southern)	Resistant cultivars Crop rotation Burial of crop residue
	Downy Mildew	Metalaxyl/mefenoxam seed treatment Do not plant on land subject to flooding Do not plant sweet corn following sorghum Two year crop rotation
	Common Smut	Resistant cultivars
	Maize Dwarf Mosaic Virus	Resistant or tolerant cultivars Eradication of johnsongrass in and around the field
Eggplant	Southern Blight	Deep burial of crop residue Crop rotation (do not follow beans, tomatoes, southern peas, okra or peanuts)
	Verticillium Wilt	Soak seed for 20 minutes in 120°F water
	Phomopsis Blight	Resistant cultivars Pathogen-free seed Three year crop rotation Apply fungicides as needed
Lettuce	Bacterial Soft Rots	Plant in well-drained soil Use furrow or drip irrigation
	Downy Mildew	Resistant cultivars Three year crop rotation Apply fungicides as needed
	Bottom Rot	Do not plant lettuce following tomatoes, Irish potatoes, or beans Plant on wide, raised beds Deep burial of crop residue Apply fungicides as needed
	Sclerotinia Drop	Follow long rotations Plant on well-drained soil Use furrow or drip irrigation; Resistant cultivars Apply fungicides as needed
	Lettuce Mosaic	Control weeds in and around the field Plant only Mosaic Indexed seed (MTO) Resistant cultivars
Mustard, Turnip, Radish	White Rust & Downy Mildew	Apply fungicides as needed Crop rotation Burial of crop residue
Mustard, Turnip, Radish	Anthracnose & Cercospora Leafspot	Two year crop rotation Apply fungicides as needed
Onion, Garlic, Shallot	Pink Root	Resistant cultivars
	Purple Blotch	Crop rotation Apply fungicides as needed
	Botrytis Leaf Blight	Apply fungicides as needed
Pea (English, Sugar Snap, Edible Pod)	Fusarium and Pythium Root Rot	Crop rotation
Pea (English, Sugar Snap, Edible Pod)	Powdery Mildew	Apply fungicides as needed Resistant cultivars
Pepper (Bell and Hot)	Phytophthora Blight	Plant on well-drained soil Plant on raised beds Apply fungicides as needed
	Cercospora Leafspot	One year crop rotation Apply fungicides as needed
	Powdery Mildew	Apply fungicides as needed
	Viruses (several)	Control weeds in and around field Resistant cultivars
	Bacterial Spot	Resistant cultivars Apply copper bactericides
Potato	Black Leg	Avoid excessive irrigation Avoid washing seed potatoes
	Common Scab	Maintain high soil moisture before and after tuber set
	Early Blight	Apply fungicides as needed
	Late Blight	Preventative applications of fungicides
	Viruses	Plant virus-free seed
Spinach, Swiss Chard	Fusarium Wilt	Long crop rotation
	White Rust	Three year crop rotation Apply fungicides as needed Burial of crop residue Resistant cultivars
	Downy Mildew	Crop rotation Soil treatment with metalaxyl or mefenoxam Resistant cultivars Burial of crop residue
	Fungal Leafspots	Apply fungicides as needed Crop rotation Burial of crop residue
	Viruses (several)	Resistant cultivars Control weeds in and around the field
Sweetpotato	Scurf	Do not use manure where sweet potatoes are to be planted Plant slips from disease free roots Crop rotation
	Southern Blight	Deep burial of crop residue Crop rotation
	Black Rot	Plant clean slips clipped 1 inch above the soil line Crop rotation
Tomato	Fusarium Wilt	Resistant cultivars
	Southern Blight	Crop rotation Deep burial of crop residue Apply soil fungicide as preplant treatment
	Early Blight	Apply fungicides as needed
	Septoria Leaf Spot	Crop rotation
	Anthracnose	Resistant cultivars Do not plant tomatoes following cabbage, lettuce, mustard and solanaceous weeds Apply fungicides as needed
	Stemphylium Leaf Spot	Apply fungicides as needed
	Late Blight	Apply fungicides as needed Use plastic mulch to prevent fruit from coming in contact with the soil Avoid heavy irrigations just prior to and during harvest
	Buckeye Rot	Three year crop rotation Plant on raised beds Avoid heavy applications of water just prior to and during harvest
	Tomato Spotted Wilt	Control thrips Rogue diseased plants Do not plant tomatoes near Irish potatoes
	Other Viruses	Resistant cultivars Control weeds in and around field