Ch. 9: Plant Disease Management

what does management imply?
use the minimum amount of action to get an adequate amount of yield to make $ and harm the environment as little as possible
what does control imply?
plants are treated as populations.
Just make the disease go away completely. Not economically or environmentally sustainable
is management of plant disease variable?
yes, it is variable with regards to the host, the pathogen, and the interaction b/w the two
when are plants treated as individuals with regard to disease?
when the host is a tree, an ornamental, or a virus-infected plant
(the exception, normally as population)
what is the most effective way to manage monocyclic pathogens?
reducing the initial inoculum is most effective b/c the amount of initial inoculum has the largest affect
What are some control measures that exclude the pathogen from the host?
1. quarantine
2. crop certification
3. Evasion or avoidance of pathogen
4. use of pathogen-free propagating material
5. minimize the possibility and risk
what does quarantine entail?
it is legal/regulatory actions, used for the most destructive pathogens, for those that do not occur in our country, and that our crops don’t have resistance to
Need good ID techniques
works for pathogens brought by humans
how long are hosts quarantined for?
40 days
where do inspections occur?
at port of entry or in some cases in the country of origin
what is an example of a plant being tested in country of origin?
Geranium: Geranium Wilt in Africa and Central America
-bad pathogen to potato’s as well
What is an example of a foreign plant that is regulated/checked for pathogens?
Holland inspects all bulbs before importation for a wide array of pathogens; it is not uncommon to have pre-entry inspections these days
what is crop certification?
use of pathogen-free stock or seed, which involves inspection and/or indexing of crops in the field or storage areas by the State agency
Where are potato seed stock grown pathogen free?
Need grow seed stock away from area of large production
How are seeds checked for pathogens?
ELISA or indicator plants for viruses, selective media for bacteria and fungi
what is ELISA?
enzyme-linked immuno-sorbent assay.
It identifies co-protein of virus by giving a color reaction related to the amount of virus in the plate
What is an example of the use of pathogen-free stock or seed?
Lettuce mosaic virus in lettuce is a big problem. Farmers are only allowed to use seed with less than 1 out of 30,000 seeds infected that will not result in significant levels of disease and loss
how are pathogen-free seeds grown?
-produced in dry, irrigated areas or remote and cool regions
-the seed crop is grown in isolated areas that are less likely to come into contact with the pathogen… i.e. evasion or avoidance of the pathogen (in greenhouses or screenhouses)
what are vegetatively propagated crops?
crops planted as tubers, bulbs, roots, crowns, seedlings, etc.
-tolerance is usually 0 (Potato)
What are Tolerance Levels? Who sets them?
The number of infected/infested plants for each of these types of crops, that are based on the max tolerances for the disease. Maximum tolerances are set by State regulatory agencies for each crop.
what are some methods of eliminating the pathogen on seeds?
-hot water treatment: can eliminate some pathogens (black rot of cabbage)
-meristem culture: used to free planting stock of pathogens (verticillium wilt of carnations)
For potato, which diseases must be entirely absent from seed to be sell-able under certification?
Ring rot and Late Blight, and nearly Root knot nematode
Does hot water treatment of seed remove all pathogens?
No, but it removes some (black rot of cabbage)
what is meristem culture? How do you use meristem culture to get healthy plants from virus-infected plants?
Cut off the apical meristem of a plant and grow these cells.
They grow the quickest, and these cells get diluted the quickest (replicate faster than virus)
then medium for roots, then medium for shoots, and test for virus until you have a mother plant (path free)
What’s an example of a plant(s) that meristem culture can be used for?
Carnations and Chrysanthemums
what are some cultural methods for eradicating or reducing the pathogen inoculum?
1. roughing
2. crop rotation to non-host crops
3. sanitation
what is roughing?
removal of the plant population and burning of them
works well in nurseries and greenhouses
Whats the deal-eo (lol) with citrus canker?
in 1915, 3 million citrus tree were roughed to eradicate the pathogen.
From 1998 to today, several billions dollars of citrus trees were destroyed due to the pathogen
-Scaly disease that affects the fruit industry, not juice
what about witchweed?
parasitized corn, sugarcane, and sorghum from Maine to Florida
what is an example of eradicating an alternate host?
eradicating barberry to prevent stem rust to wheat b/c barberry is an alternate host.
this didn’t work b/c spores still come from Mexico.
what is a soil invader?
it would rather be in the host, but will go out in the soil
what is a soil inhabitant?
it is very happy in the soil, but will invade the plant
when does crop rotation work best?
-against soil invaders
-pathogens that have a limited host range
what is sanitation and what are some examples?
eliminating or reducing the amount of inoculum present in a plant, field, or warehouse and preventing the spread of a pathogen to healthy plants
-raking infected leaves
-pruning dead branches
-washing hands (TMV)
-disinfesting pruning tools
-cutting knives
-washing soil and debris off equipment
What is an example of crop rotation to diminish disease?
The Cyst Nematode parasitizes roots of soybean, and overwinters in cysts that stay on the roots for anywhere from 1-4 years. Rotate: soybean=> corn => peanut
what does it mean to disinfest a plant?
to remove surface contamination
what does it mean to disinfect a plant?
use almost only chemical treatments (??)
What is an example of a way to eradicate or reduce the pathogen inoculum of apple scab?
Remove leaves from orchard
what are some environmental cultural methods to reduce pathogen inoculum?
-ensuring good drainage of soils to make environment less conducive for water loving pathogens like oomycetes
-flooding to eliminate nematodes
-planting density/direction of rows
how can traps and mulches be used to reduce pathogen inoculum?
-sticky traps can keep aphids away
-reflective mulches distract insect vectors (aphids)
-plastic mulches to heat soil to kill pathogens
what is pythium damping-off?
in the greenhouse soil, there is too much water in the substrate. Stems turn to goo.
what are some physical methods that reduce the pathogen inoculum?
-soil sterilization
-hot water treatments
-drying of plant surfaces
-disinfestation or disinfection
what is soil sterilization?
to kill all living organisms in soil by heat or chemicals.
-probably never true, more like pasteurization.
-can alter the soil structure
-uses steam heat and fumigates for a couple hours every couple days (3-4 times)
what are the different temperatures for different pathogens that soil is sterilized at?
viruses: 80 C
bacteria and fungi: 60 C
usually everything is killed b/w 80-90 C
what is pasteurization?
uses lower heat or shorter times to kill all or most of the “bad” microorganisms, allowing some “good” ones to live
-this avoids altering soil structure and releasing toxic levels of Mn salts and ammonia
What is standard procedure for pasteurization?
Soil is typically heated at 82 C or above for 30 minutes with steam and this kills almost all plant pathogens, weed seed and insects. This time and temperature typically won’t kill saprophytes, some heat tolerant pathogens like TMV.
what/when is hot water treatment used?
it is used for seeds and bulbs to kill pathogens infecting the tissue and can be used to disinfect or disinfest.
Temp and time is critical as too much can kill seed or bulb.
what are some examples of hot water treatment?
-loose smut of cereals
-bacterial black rot of cabbage
-kills viruses in: strawberry roots, roses, and raspberry (In cells)
what does drying of plant surfaces do? /how is it used?
Dry storage (<12% moisture for grains and legume seed) will cause bacteria and fungi to decay and disease to desiccate -many fruits are dried before storage too, to help prevent disease
what does refrigeration do to prevent disease?
-the most widely and effective method of controlling ‘post harvest’ disease of fleshy fruits and veggies
-b/w 45 and 55 F
-slows down disease progression
-hydro-cooling: removes field heat quickly
how does radiation help prevent pathogens spreading?
In the world (minus the US), various types of UV light, x-rays, gamma rays are used to kill plant pathogens on fruit and veggie products
-can do some damage to plant tissues though
what is disinfestation?
surface sterilization of crops using a strong chemical like bleach, copper sulfate, or gas
-done at 100% RH and 75-80 F
-Done to warehouses and packing crate and equipment
what are the main biological methods to reduce the pathogen inoculum?
-suppressive soils
-antagonistic organisms
-trap plants
-antagonistic plants
where does biocontrol work best? and what is it?
works best in greenhouses
-is the total or partial control of a population of plant pathogens
-occurs routinely in nature too
what are suppressive soils?
involves microorganisms, abiotic, or physical factors that are antagonistic to the pathogen.
-not always clear how suppressive it works: may be biotic, abiotic, or a combo
-green manures may be suppressive
how are suppressive soils used to control Pythium in Poinsetta?
-add substrates and additives to make good guys grow better and suppressive to bad guys
-works well in greenhouse
what are antagonistic organisms?
similar to suppressive soil, but better understood.
Non plant pathogen: oomycetes, fungi, some bacteria, and actinomycete bacteria infect spores of other pathogenic fungi
what is the best known fungal parasite?
T parasitizes the Rhizoctonia
what are trap crops?
a few rows of tall plants (rye, corn) are planted around the edges of bean, pepper, or squash field to intercept any incoming aphid carrying viruses.
-works for non-persistently transmitted viruses (majority)
-used to stimulate the hatching of nematode eggs, while also not being susceptible
what is the Witchweed eradication program?
when corn is planted near Witchweed seeds and the roots produce a signal of for germination, then hit it with a herbicide to kill Nematodes
What are the main methods for immunizing, or improving the resistance of the host?
-Induce SAR
-Plant defense activators
-improve the growing conditions of plants
-use of resistant varieties and multilines
-control through the use of transgenic plants
what is cross protection?
This applies when attempting to provide protection to a plant by infecting it with a mild strain of a virus to increase resistance to a more severe strain (TMV)
-uses coat proteins that uncoat and replicate when a severe strain arrives
-coat protein provides the cross protection
What is induced SAR?
Systematic Acquired Resistance (induced)
-This occurs when plants are infected by one pathogen and become more resistant to a subsequent infection by another pathogen.
What is an example of SAR (Induced)?
Fire blight of pear caused by Erwinia amylovora induces resistance to the fungus C. lagenarium
what are plant defense activators?
Injecting synthetic compounds into plants can create resistance of plants to several plant pathogens
-Induced SAR is against several pathogens in this case
-Salicyclic acid induce this in certain plants
How does improving the growing conditions of plants improve resistance of the host?
Improving the vigor of the plant can help increase resistance
This can be done through increasing: soil fertility, field drainage, irrigation, proper seed spacing, and weed control (get to an optimum level for all)
what does seed priming with inorganic salts or fine silicate clays do to germination?
it can accelerate germination and seedling growth and subsequently increase resistance to soilborne pathogens (Pythium or Rhizoctonia)
-gets the seed to a moisture level, then dry, so they are ready to be planted
Fire blight of pear caused by Erwinia amylovora induces resistance to?
The fungus: Collectotrichum lagenarium
-Harpin (compound) ‘turns on’ general defense system in pear
How does using resistant varieties or multilines improve resistance of the host?
This is the least expensive, easiest, and one of the most effective ways to control plant disease in crops
-no need to spray (cost and enviro effective)
-Resistance developed to many important and dangerous diseases
Multilines and Black Stem wheat rust?
The only thing to combat rust is resistance.
Multilines has many advantages…
-don’t need international coop
-can change the mixture of resistance genes from year to year
-resistance genes for the next season are determined by plant pathologists (rust gardens
what are rust gardens?
they are wheat cultivars with different genes for rust resistance planted throughout the US wheat belt
By what mechanism have chestnuts begun to recover from chestnut blight?
Infection of the pathogen by a virus-like dsRNA that limits virulence of the pathogen
What effect does Agrobacterium radiobacter have on Agrobacterium tumfaciens?
It counteracts the gall-forming tendencies of crown gall by secreting a compound known as a bacteriocin into the rhizosphere that has a deleterious effect on A. tumafaciens. Commercial strain: Galtrol
what are different types of treatments of chemicals?
-Foliage sprays and dusts: surfactants
-Seed treatments: dusts and slurries
-Soil treatments: drench (liquid) and Fumigation (gas beneath soil)
-Treatment of wounds: sterilization and dressing
-Post Harvest application: Cl water or Sulfur
Do organic or inorganic chemicals break down quickly in the environment?
Organic–inorganic are broadly toxic and do not break down quickly
What was the first inorganic fungicide? What was the first commercial inorganic fungicide?
Sulfur; Copper sulfate and lime (Bordeaux Solution)
What are antibiotics?
substances produced by one microorganisms specific for another
-not used in open field b/c resistance can develop
-limited to greenhouses
What is IPM or IPD or IPDM?
these are programs that use multiple control strategies to try to manage a plant disease problem
-try to use fewer pesticides to get the optimum result
-Utilizes: scouting, weather monitoring, and disease prediction
-reduce or eliminate initial inoculum (and it’s effectiveness)
-increase host resistance
what are nematicides?
They are a very broad spectrum of volatile soil fumigants that are biocides and are used to kill everything where applied
-Halogenated hydrocarbons
what are organic fungicides?
they degrade more easily and are more popular in the environment.
-organic Sulfur compounds were the first and most widely used organic fungicides
-Organophosphates : against oomycetes
-Pyrimidines: against rusts
-Triazoles: wide range of soilborne and root pathogens
-Aromatic: long lasting and broad spectrum
what is an inorganic chemical?
they are broadly toxic and do not break down in the environment quickly
1st: Sulfur
Most important: Bordeaux solution or CuSO4 and lime
How to apply chemicals?
Dusters: powder (not as popular now)
Sprayers: broadcast or aerial (more common)
Fumigators: Inject into the soil, bioagent: kills everything (gas or liquid)
what is a multiline?
crop may be of a single cultivar, but many different genotypes are used for resistance