Transcript Development of Disease

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.
Significance
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.
Symptoms
 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.
The Pathogens
 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.
Development of Disease
 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.
Control
 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.
小结
• 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
7.1.2 Cruciferous virus diseases
Significance
 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.
Symptoms
 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.
The Pathogens
• Turnip mosaic virus (TuMV)
 the most important and widespread virus
infecting crucifers.
 The host range for this virus is not limited to
crucifers; 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
is distributed mainly in the temperate regions of
the world.
 It is also one of the few plant viruses containing
doublestranded DNA (deoxyribose nucleic
Control
 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
小结
• 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
7.1.3 Downy mildew
Significance
 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.
Symptoms
 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 .
Bright yellow downy
mildew lesions on the
upper
surface of a diseased
turnip leaf.
Underside of diseased
turnip leaf showing
white mildew growth.
The Pathogens
Peronospora parasitica
Development of Disease
 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.
Control
 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.
小结
• 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
7.1.4 Black rot of crucifers
Significance
• 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.
Symptoms
• 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 .
Brown and yellow,
V-shaped necrotic
lesions of black rot
on the edge of a
cabbage leaf.Note
the blackened veins.
Entire cabbage leaf
showing
several 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.
The Pathogens
• Xanthomonas campestris
pv.campestris Dowson
Development of Disease
• 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 )
Control
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.
小结
• 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