4.2 Monitoring and prognosis models for diseases

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Course: Training Manual for Plant Protection in Organic Farming
Book: 4.2 Monitoring and prognosis models for diseases
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Date: Monday, 23 December 2024, 5:03 AM

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Monitoring and prognosis models for diseases


Learning Outcomes:


  • Classify diseases based on their morphology and damage pattern
  • Identify diseases based on their morphological characteristics and symptoms of damage.
  • Coordinate and organize the monitoring of diseases, identify them and decide on measures to be taken to maintain yield and prevent economic damage under certain conditions of agricultural production

Monitoring of diseases


Monitoring refers to the surveillance of processes in agricultural crops to obtain data and knowledge on diseases. Disease is assessed visually for obvious symptoms and by infestation frequency (percent of plants infested), and infestation severity (percent of plant tissue infested). The distribution pattern in the field stand is also important. There is also the possibility of an infestation survey for early detection of disease without visible symptoms. Here, random sampling is analyzed in the laboratory for pathogen genetic material using PCR testing.

Monitoring is carried out personally. Here, many years of experience on the farm and the right timing of the control play an essential role. In disease-sensitive periods or when the weather is conducive to disease, it is even advisable to carry out checks several times a day. Alternatively, consultants assist in crop inspection.

In addition, public warning services document first occurrence, infestation intensity and damage thresholds for the main cultivation areas of a crop variety in a country or region. Additional information on disease occurrence can be obtained from official advisory services.

Warning services are based on forecasting models. They are adapted to the respective climate zones and have been established for many years. Their values are based on the interaction of weather data, growth stages, infestation pressure in the region or previous year's infestation and variety susceptibility. Weather stations distributed across the country measure precipitation, humidity, air pressure, sunshine hours and wind. Based on these weather data, constantly updated and easily understandable models are created by the Plant Protection Warning Service for viticulture, orcharding, arable farming and horticulture and processed in graphs.

For example:

In viticulture, Plasmopara viticola and Erisyphe necator- pressure are calculated from the parameters humidity and atmospheric pressure

In orchards, there are very good forecasting models for the bacterial disease fire blight (Erwinia amylovora; precipitation, blossom stage) and the fungal disease scab (Venturia inaequalis; all climatic parameters, previous year's infestation, variety). For many other diseases, a risk can be well estimated: the fungal peach leaf curl (Taphrina deformans) has its germination window in the bud stage and must be controlled at this time. Bacterioses such as Pseudomonas occur after frost (microcracks) or after leaf fall (wounds).

For arable crops prediction models particularly for cereal diseases such as rust fungus, powdery mildew, and Septoria, among others, are available. Pre-harvest monitoring and early warning systems for mycotoxins in cereals and maize enable crop quality to be assured through timely fungicide application. For powdery mildew diseases other than in cereals, good empirical data on the temperature-humidity combination are available. For potato, recommendations for optimal late blight (Phytophthora) control can be calculated

In addition, for certain diseases, computer programs have been developed for farmers that use weather data to show scenarios for infestation development. Crop- and country-specific technical literature is also available.

Typical symptoms caused by bacteria, fungi, viruses


The signs of diseases are called symptoms.
Symptoms can be local - occurring on individual plant parts - or can affect the entire plant (systemic). Local symptoms include physiological changes in plant structure such as leaf spots and proliferation. Systemic changes manifest as discoloration (e.g., yellowing) or growth changes (compression, broom growth). Symptoms may be primary or secondary regarding their mode of action. Primary symptoms are directly due to the interaction of the pathogen with plant tissue (proliferation). Secondary symptoms are a result of the pathogen's activity. Parts of the plant or the entire plant are affected. An example is wilting of the entire plant due to obstruction of the conduits in the roots by soil-borne fungi in horticulture (Verticillium, Fusarium). he plant or the entire plant are affected. An example is wilting of the entire plant due to obstruction of the conduits in the roots by soil-borne fungi in horticulture (Verticillium, Fusarium).

Symptoms can be microscopic or macroscopic. While microscopic changes are identified by specialists under the microscope, macroscopic symptoms are readily identifiable during visual crop inspection (Table 4.1.).

Table 4.1. Typical macroscopic symptoms caused by bacteria, fungi, viruses (rough symptom classification)
Pathogen group overview
Symptoms Pathogen and Scientific name of the disease Transfermode of the disease, Note Examples

Local: mycelium, fruiting bodies and pustules, leaf spots, discoloration.

Systemic: wilt, dieback

Fungi

Mycosis

by wind and water (splashing)
Pay attention to warning services!

Powdery mildew, downy mildew, Fusarium, Botrytis

Viticulture: Erysiphe necator, Esca, Plasmopara viticola

Orchards: pear rust, monilia, scab

Arable crops: Phytophthora, Septoria,Rust

Horticulture: Rhizoctonia, Verticillium, Phytium, Alternaria

Brown spots on leaf, flower or fruit

Systemic: Wilt, dieback

Bacteria and phytoplasmas

Bacteriosis

Phytoplasmosis

by wind, water, vectors and contaminated work equipment.
Observe forecast models and climate data

Viticulture: bacterial leaf spot, mildew

fruit growing: fire blight, pear decay, curl disease

Arable crops: Erwinia, Streptomyces, Stolbur, stone blight, bacterial blight, tuber blight

Horticulture: Clavibacter bacterial wilt

Local: chlorotic spots, rings, necroses

Systemic: dwarfism, stunted growth, yellowing, wilting, death

Virus

Virosis

by vectors (aphid, cicada, beetles, fringed aphids, nematodes), contaminated plant material (rootstock, pollen, seeds, tubers), and contaminated implements

Viticulture: AMV, ArMV

Fruit growing: apple mosaic virus, broomrape, bark canker

Arable crops: Y- and S-virus, PNYDV (nanovirus)

Horticulture: TSWV, CABYW

Pay attention to warning services!


Table 4.2. Symptom description of the most important/frequent pathogens in viticulture
Early symptoms Late stage and other associated symptoms Pathogen
Rough symptom classification and example picture
showing typical symptoms
Description Rough symptom classification Description
Leaf spots

Leaf spots

© biohelp

Bright spots on the top side of leaves that appear dark when backlit

Fungal growth

Whitish gray fungal mycelium layer on underside of leaves

Fungus:Downy mildew (Peronosporales): Plasmopara viticola

Brown spots on leaf, flower or fruit

Superficial gray coating; spreading to the whole plant

Fungus:Gray mold : Botrytis

Chronic: Irregular yellow spots between leaf veins

Death

Leaf necrosis

Acute: Death of the plant

Fungus:Esca (fungi complex)
Powdery cover

Powdery cover

© biohelp

Whitish-gray fungal mycelium layer on lower leaf surface. Later like powdery white lawn "powdery mildew" on the upper side of the leaf.

Fungal growth

Infestation of the previous year: Erysiphe necator figures on 2-year-old wood (extensor-shot)

First symptoms of infestation on pointer shoots (= infected stingy shoots)

Infestation of the grape: fungal edge on ridges (stems) spreads from there over the berries, seed breakage as a consequence

Fungus: Powdery mildew: (Erysiphe necator)

Discoloration and growth deformation

Discoloration and growth deformation.

© biohelp

Delayed budding in spring, partial or complete yellow discoloration of the leaf blade, various leaf deformations, shortened internodes and zigzag growth.

Weak growth, scimitar, broom growth; changes and abnormal branching of the vine wood, small berries and increased trickling.

Virus: Grapevine fanleaf virus + Arabis mosaic virus

The oldest leaves curl downward, at the same time the leaf blades begin to turn yellow (white wine varieties) or dark red (red wine varieties) from the edges

In the final stage, only the main veins with their fringes remain green; symptom continuation along the shoots. Growth depression; increased trickling of the shoots

Virus: Grapevine leafroll virus GLRaV 1+3

Light coloration of leaf veins in young leaves

Mosaic-like patterns on older leaves, curvature

Virus: Marbling of the vine (Grapevine fleck virus GFkV)

Growth deformation: crippled shoots, shortened internodes, deformed, chlorotic leaf spots

Reduction quality and quantity of the yield

Virus: Ruländer disease (Grapevine Pinot gris virus)

Stem and branch changes

Virus: Wood rot, corky bark disease (Grapevine virus A + B)


Table 4.3. Symptom description of the most important/frequent pathogens in orchards
Early symptoms Late stage and other associated symptoms Pathogen Culture
Rough symptom classification and example picture
showing typical symptoms
Description Rough symptom classification Description
Leaf spots

leaf spots

© biohelp

Bright spots on the top side of leaf

Fungal growth

White fungal mycelium mostly on the top side of leaves - can be easily wiped off; spread to all parts of plant; stunted growth, brown discoloration and drying of leaves/plant

Fungus:Powdery mildew (Erisiphiaceae)

Orchards

Bright spots on the top side of leaves that appear dark when backlit

Whitish gray fungal mycelium layer on underside of leaves

Fungus:Downy mildew (Peronosporales) - species group!

Orchards

Brown spots on leaf, flower or fruit

Superficial gray coating; spreading to the whole plant

Fungus: Gray mold (Botrytis cinerea)

Orchards (strawberry)

Rust colored spots

Pustules

Pustules on leaves, breaking open pustule-shaped spore deposits; death of plant parts

Fungus:Rust fungi (Pucciniales)

Apple, pear, plum

Dull olive green, later brown or humped blackish spots on leaves

Necroses

Coalescence of spots, necrosis, leaf fall,cracked corked fruit skin

Fungus: Apple scab (Venturia inaequalis)

Apple

Small, angular, watery spots bordered by the leaf veins, appearing translucent in backlight and black in incident light

Mucus secretion and death

Symptom spread to whole leaf, leaf dieback, mucus discharge

Bacterium: angular leaf spot (Xanthomonas fragariae)

Strawberry

Shot-like, translucent symptoms with yellowish border

Lesions

Grooved, sunken, black-red lesions on bark of trunk and branches

Bacterium:Bacterial canker (Pseudomonas syringae pv. morsprunorum)

Stone fruit (plum, cherry)

Discoloration and growth deformation

Discolouration and deformation

© biohelp

Curling and blistering of young leaves with partial red discoloration

 

Severe curling of leaves, chlorosis; reduction of fruit 

Fungus:Curl disease (Taphrina deformans)

Peach, Nectarine

Brown/black coloration and wilting from the petiole, bending of the shoot tips

Mucus secretion and death

Leakage of bacterial slime, death of the plant between a few weeks (young plants) and a few years

Bacterium: Fire blight (Erwinia amylovora)

Pome fruit

Premature sprouting, chlorotic leaf rolling

 

Early leaf drop, Necrosis of phloem, Abnormal fruit development and early fruit drop

Phytoplasma:European Stone Fruit Yellows (ESFY, Candidatus phytoplasma prunorum)

Stone fruit

Early shoots with red leaf tips, stipules enlarged

 

Autumn color already in summer, broom-like branching of one-year-old shoots "witches broom"

Phytoplasma: Apple shoot blight (Candidatus phytoplasma mali)

Apple

Spots, ring spots (leaf, fruit, core), broom growth

   

Virus: E.g.: Cherry: hundreds of virus species, Raspberry 280 viruses)

Orchards

Wilt

Wilt

© biohelp

Shoot tip dieback

 

Gum flow; fruits turn brown, dry up and show white fruit bodies

Fungus: Monilia (Monilia spp.)

Orchards

Withered leaves

Death

Death of the entire plant

Fungus; soilborne: Verticillium wilt (V. dahliae)

Raspberry, Strawberry, cherry


Table 4.4. Symptom description of the most important/common pathogens in arable crops
Early symptoms Late stage and other associated symptoms Pathogen Culture
Rough symptom classification and example picture
showing typical symptoms
Description Rough symptom classification Description
Leaf spots

leaf spots

© Penn State Dpt. of Plant Pathology

Bright spots on the top side of the leaf

Fungal growth

White fungal mycelium mostly on the top side of the leaves- can be easily wiped off; spread to all parts of plant; stunted growth, brown discoloration and drying of leaves/plant

Fungus:Powdery mildew (Erisiphiaceae)

Arable farming

Brown irregular spots on leaves

Brown spots on stems and fruits; White-gray fungal mycelium on underside of leaves; Rotting or withering of leaves, rotting of fruits and tubers

Fungus: Downy mildew: Late blight; (Phytophthora infestans)

Tomato, potato

Brown spots on leaf, flower or fruit

Superficial gray coating; spreading to the whole plant

Botrytis, gray mould (Botrytis cineraria)

Arable farming

Rust colored spots and pustules on leaves

Pustules

Breaking open pustule-shaped spore deposits; death of plant parts

Fungus:Rust fungi (Pucciniales)

Cereal, asparagus, bean, pea, leek

Leaf spots

leaf spots cereal

© biohelp

Oval, yellow-green, chlorotic spots on the lower leaves

Necroses

Gray-green streaky necrosis, leaf drought, black fruiting bodies on upper and lower leaf surfaces

Fungus: Septoria leaf spot (Septoria tritici)

Cereals

Wilt

wilt

© biohelp

Fading and/or wilting of leaves or fruit clusters

Sporenbelag

Discoloured spikelets and orange coloration of lemmas due to spore coatings in cereals; yield reduction

Fungus; soilborne: Fusarium head blight and stem and ear rot

Cereals, corn

Withered leaves

Death

Death of the entire plant

Fungus; soilborne: Verticillium wilt (V. dahliae V. longisporum on cabbage)

Sugar beet, hops, sunflower. Peas, beans, cabbage

Withered leaves

Death

Death of the entire plant

Fungus; soilborne: Fusarium wilt 

Onion, cabbage, spinach, cucumber, pea, bean

Necrosis and strangulation of seedlings, wilting symptoms

Death

Overturning of seedlings, death of above- and below-ground plant parts

Fungus; soilborne: Rhizoctonia falling sickness, stem rot (R. solani, R. sp.), beet rot (R. solani AG 2-2)

Arable farming

Discoloration and growth deformation

Discoloration and growth deformation arable

© Howard F. Schwartz, Colorado State University, Bugwood.org

White stem base (whitelessness), curling of the top leaves

 

Air nodules, nodule deformation, Dry core

Fungus; soilborne: Rhizoctonia(R. solani AG 3)

Potato

Chocolate-brown necroses on the leaves (drought spots, spray spots)

Necroses

Coalescence of necroses, destruction of leaf mass, partial stains on stems

Fungus; soilborne: early blight (Alternaria sp.)

Potato

Growth deformation

Growth deformation arable crops

© Central Science Laboratory, Harpenden , British Crown, Bugwood.org

Cauliflower-like cell growths on the tuber

Spore covering

Black spore powder from growths; yield loss, crop failure

Fungus: Potato canker (Synchytrium endobioticum)

Potato

Smell
smell

© Howard F. Schwartz, Colorado State University, Bugwood.org

Fishy odor in wheat

Spore covering

Instead of spikes, black-brown spore deposits are formed

Fungus: stinking smut (Tilletia spp.)

Cereals

Rot
Rot arable

© Howard F. Schwartz, Colorado State University, Bugwood.org

Muddy, slimy rot on tuber, turnip, stem or leaves, unpleasant odor

Death

Complete rotting of the stored part of the plant

Bacterium: Bacterial, tuber wet rot, soft rot, blackleg of potato (Pectobacterium carotovorum)

Potato,carrot, cabbage, celery

Discoloration and growth deformation
Discoloration and growth deformation arable

© biohelp

Leaf yellowing, dwarfism, leaf deformation and curled leaves

Necroses

Necrosis, dieback

Virus: Pea necrotic yellow dwarf virus (PNYDV)

Native legumes. alfalfa and soybean not affected!

Discoloration, necrotic spots on leaf and stem

 

Flesh discolored such brown

Virus:  tobacco rattle virus (TRV)

Potato


Table 4.5. Symptom description of the most important/common pathogens in horticulture
Early symptoms Late stage and other associated symptoms Pathogen Culture
Rough symptom classification and example picture
showing typical symptoms
Description Rough symptom classification Description
Leaf spots

leaf spots horticulture

Bright powdery spots on the top side of the leaf

Fungal growth

White fungal mycelium mostly on the top side of the leaves - can be easily wiped off; spread to all parts of plant; stunted growth, brown discoloration and drying of leaves/plant

Fungus:Powdery mildew (Erisiphiaceae)

Horticulture

Bright spots on the top side of leaves

Whitish gray fungal mycelium layer on underside of leaves

Fungus:Downy mildew (Peronosporales) - species group!

Horticulture

Brown irregular spots on leaves

Brown spots on stems and fruits; White-gray fungal mycelium on underside of leaves; Rotting or withering of leaves, rotting of fruit

Fungus:Late blight (Phytophthora infestans) - type of downy mildew!

Tomatoe

Brown spots on leaf, flower or fruit

Superficial gray coating; spreading to the whole plant

Fungus: Gray mold rot (Botrytis cinerea)

Strawberry, Cucumber

Leaf spots

leaf spots 2

© biohelp

Yellowish, blurred brightenings on the upper side of the leaf

Fungal growth

Patches of gray-brown to olive-green mycelial coating on the underside of leaves

Fungus: Velvet spot 

Tomato

Slight lightening of the veins, faintly visible concentric rings on the leaf, stem and fruit. Depending on the variety, the color and shape of the spots differ

Growth deformation

Fruit deformation

Virus: Tomato spotted wilt virus (TSWV)

Tomato, bell pepper

Discoloration and growth deformation
Discoloration and growth deformation horticulture

© biohelp

Older leaves become chlorotic, leaf thickening and brittle leaves

 

Entire plant may become chlorotic, Reduced fruit set, Fruit shedding

Virus: Cucurbit aphid-borne yellows virus (CABYV)

Horticulture

Wilt
wilt horticulture

© biohelp

Withered leaves

Death

Death of the entire plant

Fungus; soilborne: Verticillium wilt (V. dahliae and V. albo-atrum)

Cucumber, tomato

Welken von B Withered leaves lättern

Fungus; soilborne: Fusarium wilt 

Discoloration and strangulation of seedlings and young plants at the root neck

Fungus; soilborne: Pythium root rot (Pythium sp.)

Horticulture

Necrosis and strangulation of seedlings, wilting symptoms

Overturning of seedlings, death of above- and below-ground plant parts

Fungus; soilborne: Rhizoctonia damping-off (R. solani, R. sp.)

Leaf parts wilting

Dieback of plant parts, bird's eye spots on fruit

Bacterium: Bacterial wilt (Clavibacter michiganensis)

Tomato, bell pepper