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) |
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| 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 |
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| 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) |
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| 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) |
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| Light and dark green areas on a leaf | Appears in a random pattern in the field (Virus) Appears in rows (Nutrient Deficiency) |
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| Leaves distorted | Virus Herbicide |
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| 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) |
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| 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) |
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| Root tips dead | Isolated areas (Nematodes; Fertilizer Burn) Affected area is in a pattern or over the general field (Fertilizer Burn) |
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| 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 |
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| Bean Mosaic (virus) | Virus-free seed Resistant cultivars |
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| Bacterial Blight | Pathogen-free seed Apply copper fungicides Two year crop rotation |
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| Cercospora and other Fungal Leafspots | Apply fungicides as needed Two year crop rotation |
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| White Mold | Apply fungicides as needed Wide row spacing and orientation to favor drying of the canopy |
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| Rust | Apply fungicides as needed Crop rotation Resistant cultivars |
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| 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 |
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| 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 |
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| Downy Mildew | Resistant cultivars Apply fungicides on a preventative basis |
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| Anthracnose (watermelon, cucumber, cantaloupe) | Resistant cultivars Crop rotation Apply fungicides on a preventative basis |
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| Alternaria Leafspot, Cercospora Leafspot | Apply fungicides as needed Two year crop rotation |
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| Angular Leafspot | Pathogen-free seed Apply copper fungicides Avoid overhead irrigation |
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| Bacterial Wilt (cucumber) | Control beetle vector Resistant cultivars |
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| Fruit Blotch (watermelon) | Pathogen-free seed Copper fungicides Two year crop rotation Avoid overhead irrigation |
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| Gummy Stem Blight | Two year crop rotation Apply fungicides on a preventative basis |
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| Choanephora Wet Rot | Frequent sprays of blossoms with copper fungicides Keep fruit cool and dry after harvest |
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| 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 |
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| 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 |
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| 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 |
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| Common Smut | Resistant cultivars | |
| Maize Dwarf Mosaic Virus | Resistant or tolerant cultivars Eradication of johnsongrass in and around the field |
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| 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 |
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| 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 |
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| 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 |
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| Sclerotinia Drop | Follow long rotations Plant on well-drained soil Use furrow or drip irrigation; Resistant cultivars Apply fungicides as needed |
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| Lettuce Mosaic | Control weeds in and around the field Plant only Mosaic Indexed seed (MTO) Resistant cultivars |
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| Mustard, Turnip, Radish | White Rust & Downy Mildew | Apply fungicides as needed Crop rotation Burial of crop residue |
| Anthracnose & Cercospora Leafspot | Two year crop rotation Apply fungicides as needed |
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| Onion, Garlic, Shallot | Pink Root | Resistant cultivars |
| Purple Blotch | Crop rotation Apply fungicides as needed |
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| Botrytis Leaf Blight | Apply fungicides as needed | |
| Pea (English, Sugar Snap, Edible Pod) | Fusarium and Pythium Root Rot | Crop rotation |
| Powdery Mildew | Apply fungicides as needed Resistant cultivars |
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| 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 |
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| Powdery Mildew | Apply fungicides as needed | |
| Viruses (several) | Control weeds in and around field Resistant cultivars |
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| Bacterial Spot | Resistant cultivars Apply copper bactericides |
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| 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 |
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| Downy Mildew | Crop rotation Soil treatment with metalaxyl or mefenoxam Resistant cultivars Burial of crop residue |
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| Fungal Leafspots | Apply fungicides as needed Crop rotation Burial of crop residue |
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| Viruses (several) | Resistant cultivars Control weeds in and around the field |
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| 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 |
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| Black Rot | Plant clean slips clipped 1 inch above the soil line Crop rotation |
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| Tomato | Fusarium Wilt | Resistant cultivars |
| Southern Blight | Crop rotation Deep burial of crop residue Apply soil fungicide as preplant treatment |
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| 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 |
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| Buckeye Rot | Three year crop rotation Plant on raised beds Avoid heavy applications of water just prior to and during harvest |
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| Tomato Spotted Wilt | Control thrips Rogue diseased plants Do not plant tomatoes near Irish potatoes |
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| Other Viruses | Resistant cultivars Control weeds in and around field |
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