Chapter 7 Vegetables Diseases

§7-1 Crucifers vegetable diseases

7.1.1 BACTERIAL SOFT ROT Bacterial soft rots occur worldwide and cause serious diseases of crops in the field, in transit( 运输 ), and especially in storage. They cause a greater total loss of produce than any other bacterial disease. Bacterial soft rots reduce quantities of produce available for sale, reduce the quality and thus the market value of crops, and increase expenses greatly for preventive measures against soft rots.

Bacterial soft rots occur most commonly on fleshy（肉质的） storage tissues of vegetables and annual ornamentals such as potatoes, carrots, onions, iris, and fleshy fruit such as cucumber and tomato, or succulent （多汁的） stems, stalks, or leaves, such as cabbage, lettuce, celery, and spinach.

Significance

• In the tropics, soft rots often develop on the fleshy stems of some plants while still in the field, e.g., in corn, cassava, and banana.

• Nearly all fresh vegetables are subject to bacterial soft rots, which may develop within a few hours in storage or during marketing.

Significance

Soft-rot symptoms begin as a small water-soaked lesio, which enlarges rapidly in diameter and in depth.

The affected area becomes soft and mushy （糊状的） while its surface becomes discolored and somewhat depressed.

Tissues within the affected region become cream colored and slimy, disintegrating into mass of disorganized plant cells and bacteria.

Symptoms

The outer surface may remain intact while the entire contents have changed to a turbid liquid; alternatively, cracks develop and the slimy mass exudes to the surface and, in air, turns tan, gray, or dark brown.

Symptoms

A whole fruit or tuber may be converted into a sort, watery, decayed mass within 3 to 5 days. Infected fruits and tubers of many plants are almost odorless until they collapse, and then secondary bacteria grow on the decomposing tissues and produce a foul odor.

When root crops are affected in the field, the lower parts of the stem may also become infected and watery and may turn black and shrivel, causing the plants to become stunted, wilt, and die.

Symptoms

Erwinia carotovora pv. carotovora, E. chrysanthemi, and Pseudomonas fluorescens.

Bacteria E. carotovora pv. carotovora and P. fluorescens cause the most common and the most destructive soft rots.

Erwinia caratovora pv. atroseptica, the cause of blackleg of potato, may be thought of as a cool temperature variant of E. caratovora pv. carotovora and is restricted mostly to potatoes.

The Pathogens

Erwinia chrysanthemi affects many hosts and causes many of the soft rot of tropical plants while they are still growing in the field.

Soft-rot bacteria can grow and are active over a range of temperatures from 5 to 35°C. They are killed with extended exposure at about 50°C.

The Pathogens

Soft-rot bacteria survive in infected fleshy organs in storage and in the field, in debris, on roots or other parts of host plants, in ponds and streams used for water irrigation, occasionally in the soil, and in the pupae of several insects.

The disease may first appear in the field on plants grown from previously infected seed pieces. Some tubers, rhizomes, and bulbs become infected through wounds or lenticels after they are set or formed in the soil.

Development of Disease

The inoculation of bacteria into fleshy organs and their further dissemination in storage and in the field are facilitated greatly by insects.

Soft-rot bacteria can live in all stages of the insect. Moreover, the bodies of the insect larvae( 幼虫 ) (maggots) become contaminated with bacteria when they crawl about on rotting seed pieces, carry them to healthy plants, and put them into wounds where they can cause the disease.

Development of Disease

Even when the plants or storage organs are resistant to soft rot and can stop its advance by the formation of wound cork layers, the maggots destroy the wound cork as fast as it is formed and the soft rot continues to spread.

When soft-rot bacteria enter wounds, they feed and multiply at first on the liquids released by the broken cells on the wound surface.

There they produce increasing amounts of pectolytic enzymes that break down the pectic substances of the middle lamella and bring about maceration of the tissues.

Development of Disease

Because of the high osmotic pressure of the macerated tissue, water from the cells diffuses into the intercellular spaces; as a result, the cells plasmolyze, collapse, and die.

Bacteria continue to move and to multiply in the intercellular spaces, while their enzymes advance ahead of them and prepare the tissues for invasion.

The invaded tissues become soft and are transformed into a slimy mass consisting of innumerable bacteria swimming about in the liquefied substances.

Development of Disease

The epidermis of most tissues is not attacked by the bacteria; however, cracks are usually present, and the slimy mass extrudes through them into the soil or in storage, where it comes into contact with other fleshy organs, which are subsequently infected.

Development of Disease

The control of bacterial soft rots of vegetables is based almost exclusively on sanitary and cultural practices.

All debris should be removed from warehouses, and the walls should be disinfested with formaldehyde（甲醛） or copper sulfate.

Wounding of plants and their storage organs should be avoided as much as possible. Products to be stored should be dry, and the humidity and temperature of warehouses should be kept low.

Control

In the field, plants should be planted in well-drained areas and at sufficient distances to allow adequate ventilation( 通风 ).

Susceptible plants should be rotated with cereals or other nonsusceptible crops. Few varieties have any resistance to soft rot, and no variety is immune.

Chemical sprays are generally not recommended for the control of soft rots.

Control of the insects that spread the disease reduces infections both in the field and in storage.

Control

Experimental biological control of bacterial soft rot of potatoes has been obtained by treating potato seed pieces before planting with antagonistic bacteria or with plant growth-promoting rhizobacteria.

Control

• 发生概况：危害？产量损失• 病害识别：发病时期？症状特点？• 病原：病原细菌；寄主范围；致病机理• 病害发生发展规律：越冬、传播、入侵方式？• 发病及其影响因素：气象因素（雨水）；昆虫；伤口

及愈伤能力；寄主抗病性• 综合防治：选用抗病品种；加强栽培管理；及时防治

害虫；化学防治

小结

7.1.2 Cruciferous virus diseases

Many vegetables in the family Cruciferia are grown in New York, including cabbage, cauliflower, broccoli, Brusselss sprouts, Chinese cabbage, mustard greens, turnip, and radish.

Although at least six viruses are known to infect cruciferous plants, the two most important viruses are turnip mosaic virus and cauliflower mosaic virus.

Significance

TuMV causes mosaic and black necrotic ring spots in cabbage, cauliflower, and Brusselss sprouts.

Necrotic spots may not be evident on cabbage heads at harvest, but may appear after 2 to 5 months in storage.

These spots are the result of infections that occurred during the growing season. Infections do not spread among heads while in storage.

Symptoms

Spotting may be found several layers deep within the head and appears on the midribs, the side veins, and in the interveinal areas where the spots may coalesce.

This is particularly disconcerting because outwardly the heads appear normal.

TuMV causes mosaic with leaf distortion and necrosis especially on the lower leaves of turnip, radish, mustard, and Chinese cabbage .

Symptoms

CaMV is another virus capable of infecting crucifers, and its symptoms have often been confused with those of TuMV infections.

The virus induces mosaic and a striking veinal chlorosis in most of its hosts.

A masking of symptoms may occur in chronically infected plants, particularly at high temperatures. Infected plants of turnip, Chinese cabbage, and other species tend to flower prematurely.

Cabbage heads displaying "pepper spotting" and "vein streaking necrosis" in storage were previously thought to be infected by CaMV.

Symptoms

• Turnip mosaic virus (TuMV) the most important and widespread virus infecti

ng crucifers. The host range for this virus is not limited to cruc

ifers; the virus also presents problems for lettuce and endive, spinach, and several bedding plants like zinnia and petunia. T

uMV is not seedborne in any species, but is efficiently transmitted in a nonpersistent manner by several aphid species, most notably the green peach aphid (Myzus persicae) and the cabbage aphid (Brevicoryne brassicae).

The Pathogens

• Cauliflower mosaic virus (CaMV) CaMV has a host range limited to crucifers and i

s distributed mainly in the temperate regions of the world.

It is also one of the few plant viruses containing doublestranded DNA (deoxyribose nucleic

The Pathogens

Unproductive fields should be plowed-down and planted in corn, sorghum, a small grain, or forage grasses.

Plant virus-free cultivar and clover seed that is certified.

Whenever feasible, do not grow cultivar close to other Cruciferae plant.

Keep down weeds in drainage ditches and fencerows, along roadsides, and in other waste areas.

Control

The ultimate solution is to develop plant and clover varieties that are resistant to the most prevalent and injurious virus diseases.

It will be a number of years, at least, before varieties highly resistant to several viruses become available

Control

• 发生概况：分布？ 危害？• 病害识别：发病时期？症状特点？• 病原：不同毒源种类的寄主范围，传毒方式• 病害发生发展规律：不同地区病害的传播• 发病及其影响因素：介体与气候条件；品种因素；

栽培与耕作因素；生境因素• 综合防治：选用抗病品种；防治传毒蚜虫；加强栽

培管理；化学防治

小结

7.1.3 Downy mildew

Downy mildew affects all cultivated plants and weeds in the crucifer family.

It can be a serious problem in commercial production of cabbage, broccoli, cauliflower, radish, turnip, mustard, collard, and cruciferous greens.

Under favorable conditions, it may cause serious losses in the field or may develop after harvest and cause deterioration of product quality during packing and shipping.

Significance

Plants can be infected at any stage of development. In seed beds, cotyledons and primary leaves are invaded resulting in fungal growth visible on the underside of the leaf.

Later a slight yellowing develops opposite the fungal growth on the upper side of the leaf.

The young leaf or cotyledon, when yellow, may drop off. Older leaves usually persist and infected areas gradually

enlarge, turn bright yellow, then become tan and papery. Rarely the affected leaf may develop hundreds of minute

darkened specks. Under cool, moist conditions, a white mildew growth can be

seen on the underside of infected leaf lesions

Symptoms

Symptoms may appear on other plant parts as well. The fleshy roots of turnips and radishes may develop an

internal, irregularly shaped discoloration extending from the crown downward.

The flesh may be brown or black or show a form of net necrosis.

In advanced stages, the skin becomes roughened by minute cracks and the root may split open.

In radish these symptoms may be confused with those caused by Rhizoctonia .

Symptoms

Bright yellow downy mildew lesions on the upper surface of a diseased turnip leaf.

Underside of diseased turnip leaf showing white mildew growth.

Peronospora parasitica

The Pathogens

The pathogen overwinters in roots or in decaying portions of diseased plants.

Thick-walled resting spores may form in stems, cotyledons, and other fleshy parts of infected host plants.

On growing plants, the fungus produces large numbers of spores that are blown about by wind and splashed by rain.

Moisture and temperature are important in the spread and reproduction of this fungus.

Development of Disease

High relative humidity during cool or warm, but not hot, periods promotes its growth and sporulation.

Presence of a water film on the foliage from fog, drizzling rain, or dew allows spores to germinate, infect, and produce more spores on a susceptible host in as few as 4 days.

Development of Disease

Use a crop rotation plan that excludes production of any type of cruciferous crop for at least 2 out of every 3 years.

Practice sanitary measures such as the use of clean seed beds away from other crucifer production and the destruction of cruciferous weeds.

Use a planting site and plant spacing pattern that expose plants to full sun throughout the day.

Control

If severe disease pressure is expected, apply a registered fungicide weekly beginning soon after emergence.

Disease resistant cultivars are not available for most cruciferous crops. However, some hybrid cultivars of broccoli are resistant or tolerant to downy mildew.

Control

• 发生概况：分布？ 危害？• 病害识别：发病时期？症状特点？• 病原：形态特点；生理分化• 病害发生发展规律：南、北地区病害的传播的不同• 发病及其影响因素：气象因素；栽培管理；寄主抗

病性• 综合防治：选用抗病品种；合理轮作，适期播种；

加强水肥管理；化学防治

小结

7.1.4 Black rot of crucifers

• Black rot is the most serious disease of crucifer crops world wide when environmental conditions (relatively high temperature and humidity) are favorable.

• The disease affects primarily aboveground parts of plants at any stage of growth and causes high yield and quality losses, especially in tropical and subtropical regions during the rainy season.

• All vegetables in the crucifer family, including broccoli, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, kale, mustard, radish, rutabaga, and turnip, are susceptible to black rot.

• Many cruciferous weeds such as Shepherd's Purse, wild mustard, and yellow rocket may also be hosts of this pathogen.

Significance

• Seedlings grown from contaminated seed are often systemically infected, turning pale yellow and dying under warm conditions.

• Older plants are infected through vein endings at the leaf margin, initiating yellow, V-shaped lesions which turn brown and necrotic as they expand towards the base of the leaf.

• The veins of infected leaves, stems, or roots turn black as the multiplying bacteria plug the normal flow of water and nutrients.

• Black rot infection is often followed by soft rot organisms, which further reduce the quality and storage life of vegetable brassicas .

Symptoms

Brown and yellow, V-shaped necrotic lesions of black rot on the edge of a cabbage leaf.Note the blackened veins.

Entire cabbage leaf showingseveral black rot lesions.

Veins of plant systemically Infected with black rot turn dark brown to black in colour.

Blackened vascular system of black rot infected rutabaga.

• Xanthomonas campestris pv.campestris Dowson

The Pathogens

• The bacteria can overwinter in plant debris, in and on seeds from diseased plants, and in and on weeds.

• The pathogen may survive in diseased crop residue buried in soil for up to 2 years, but not more than 60 days free in soil.

• The major source of these bacteria is infected seeds, which enable long-distance spread of the disease.

• The pathogen is spread within and between fields by splashing water, wind, insects, machinery, and irrigation or drainage waters.

Development of Disease

• The bacteria infect the cotyledons and young leaves through natural plant openings (stomata, hydathodes) or wounds and then migrate between cells until they reach the xylem tissue where they spread throughout the plant. Free moisture is required for infection by the pathogen. After infection, symptoms may appear on plants within 7 to 14 days under optimum conditions (25 to 30 degrees )

Development of Disease

Effective management of black rot of crucifers depends on the application of the following practices in combination:

1. Use black rot-tested, disease-free seed grown in an arid production area.

• If source of the seeds is unknown, or infested seedlots must be used, treat seed with hot water to eradicate pathogenic bacteria. Cabbage, broccoli, and Brussels sprouts can be treated at 50 degrees C for 25 minutes, while seeds of cauliflower, kale, turnip, and rutabaga are treated for 15 minutes.

Control

• However, this treatment may reduce the viability of seed. Therefore, some other chemical seed treatments, including, sodium hypochlorite, hydrogen peroxide, and hot acidified cupric acetate or zinc sulfate can be applied to eliminate the bacteria from crucifer plant seeds.

Control

2. Use certified disease-free transplants.

3. Practice crop rotation where crucifers are grown only every 3 to 4 years to eliminate the inoculum sources from diseased crop debris in the soil.

4. Good sanitation practices should be performed to prevent disease spread.

• a. Eliminate all volunteer crucifer plants from previous crops and alternative wild host plants within and around the field.

Control

• b. Do not apply manure that may contain crucifer residues.

• c. Do not use sprinkler irrigation.• d. Avoid working in the field when plants are wet. • e. Do not allow machinery and equipment

movement from infested areas to non-infested fields.

• f. Deep plow to bury all crucifer residues after harvest.

Control

5. Application of fixed copper pesticides in the field may help to reduce spread of the disease. Consult the Ohio Vegetable Production Guide (OSU Extension Bulletin No. 672) for current recommendations.

6. A few black rot-resistant cultivars of cabbage and other crucifers are commercially available. These resistant cultivars should be used in crucifer growing regions where black rot is a common problem.

Control

• 发生概况：分布？ 危害？产量损失• 病害识别：发病时期？症状特点？• 病原：病原细菌；寄主范围；致病机理• 病害发生发展规律：越冬、传播、入侵方式• 发病及其影响因素：气象因素（雨水）；昆虫；栽

培管理；伤口；愈伤能力；寄主抗病性• 综合防治：选用抗病品种；加强栽培管理；及时防

治害虫；化学防治

小结