- Distribution of Affected Plants
- Symptoms of Common Diseases
- Signs Of Common Diseases
- Fungi (most common plant pathogen)
- Insects and Mites
- Genetic Aberration
- Guttation Injury (exudation of drops of sap on the tips or edges of leaves)
- Herbicide Damage
- Mineral Deficiencies
- Mineral Excess and Spray Damage (see also saline soils)
- Plant Hormones 2, 4-D, etc.
- Saline Soils
- Toxic Plant Residues
- Weather – Frost Injury
- Weather – Excess Water
The following is an outline of some of the causes and characteristics of the more common diseases and disease-like symptoms seen in crops and things to consider during your diagnosis.
The distribution of the affected plants within a paddock can often help point to whether symptoms are actually caused by a disease or some other factor. Take time to get a thorough overview of symptoms in the crop. Make a sketch or take a photograph.
Affected plants occurring in rows or lines could indicate:
- malfunction of equipment such as depth of seeder problems with fertiliser or herbicide applicator
- herbicide spray overlap
- infectious disease spread by contact and consistent with cultural operations
- a pathogen like cereal cyst nematode that can be dragged along the rows during cultivation.
Random individual affected plants may be caused by:
- seed-borne disease
- spores blown in from a great distance
- an insect that spreads the pathogen.
Affected plants that are more concentrated on the edges of the paddock, may be caused by:
- insect vectored disease
- spores or cysts blown in from adjacent paddocks
- pesticide drift from adjacent paddocks.
Affected plants occurring in patches could indicate:
- soil-borne disease such as cereal cyst nematode or Rhizoctonia root rot
- soil type
- a insects such as aphids, or virus spread by aphids.
Uniformly affected plants across the paddock, more likely to be caused by:
- herbicide damage
- nutrient deficiency or toxicity
- stubble-borne diseases.
When diagnosing symptoms, you’ll need to investigate the:
- internal, as well as, the external parts of the plant; attempt to determine what part of plant is being affected first
- history of symptoms
- leaves and stem(s) of affected plants
- flowers and fruits
- roots (dig up suspect plants, wash soil from roots and examine carefully – preferably in a tray of water).
The more common diseases are likely to include the presence of one or some of the following:
- fungal fruiting structure such as pycnidia
- mycelium (fungal growth) in tissue
- bacterial ooze or bacteria in tissue
- insects – mites or aphids
- nematodes – check if there are cysts on the roots
- higher plant parasites – some, for example, branched broomrape (Orobanche ramosa), are only attached to roots.
Symptoms can be extremely diverse and difficult to generalise. They may include:
- mycelium (fungal growth); often in the lesions
- fruiting structures, such as pycnidia, may be present on plants, stubble, or in the soil
- isolation from infected tissue may be necessary; beware of secondary organisms developing on tissue killed by other causes.
- leaf spot often are water-soaked; may have bacterial ooze
- presence of bacteria in young infected tissue
- there are a number of bacterial wilt and gall diseases.
This can include a great variety of symptoms, such as:
- tissue malformation and ‘hopper’ burns common
- presence of known parasitic insects such as aphids or mites
- plants usually recover after the pest has been eliminated.’
- plants are often stunted and yellowed
- galls, knots or lesions on leave or roots; the latter may be decayed or distorted in other ways
- presence of nematodes with stylets in or on tissue
- presence of cysts on the roots.
- mosaic leave symptoms similar to those caused by some viruses and some deficiencies e.g. Yellowing in Kord CL Plus, Grenade CL Plus, Justica CL Plus, Axe, Gladius and Correll wheat varieties
- disease does not spread in the field
- appears in most environments and locales
- non transmissible; often traceable to certain clones or varieties.
- tip and margin of leaf burnt
- usually occurs after a succession of warm days and cool nights
- soil abundantly moist and fertile.
Symptoms are diverse and depend on type of herbicide, often with pre-emergence treatments injuries originate at roots. Symptoms may include root thickening, generalised necrosis extending up the stem, on leaves, marginal necrosis, necrotic and chlorotic areas, cupping and shoe string. There may be also epinasty and rosetting. These latter symptoms are associated with hormonal type materials.
Patterns in the field may correlate to row treatment of previous year’s crop. Injury may vary with soil type, topography and uneven application.
Examine other plants for similar symptoms.
Various symptoms though interveinal chlorosis, (compare with photographs in Winter Cereal Nutrition: The UTE Guide available from the GRDC www.grdc.com.au/ ). Recovery usually occurs following injection or spraying with suspected deficient elements.
Compare symptoms in combination with a soil nutrient and/or a plant tissue test.
Recovery usually only occurs following injection or spraying with suspected deficient elements.
- leaves often have marginal necrosis and dull brown spotting
- excess mineral symptoms are not a diagnostic as are deficiency symptoms.
- usually profound distortions of leave
- similar symptoms on different broad-leaved species in the case of 2,4-D
- plants often recover.
Obvious symptoms may be lacking even with considerable decrease in yield
- leaves, stem and fruits are usually smaller than normal
- leaves are characteristically deeper blue-green
- damage usually occurs in arid or semi-arid regions
- presence of indicator plants that tolerate high saline soils
- test measuring electrical conductivity of soil solutions is necessary to determine degree of salinity.
- primary root of seedlings inhibited and roots have surface lesions in some cases
- excess plant residue in soil
- disease often associated with heavy, poorly aerated, waterlogged soils and relatively cool temperatures
- occurs on several unrelated species
- may be a general decrease in plant size.
- often chlorotic and necrotic leaf patterns; occasionally leaf, shoot and fruit distortions (symptoms may be masked at different times of season or disappear entirely after shock stage)
- disease often spreads in the field
- may be randomly distributed in the field or at edges of field near perennial source of virus
- often on a variety of sites
- absence of known toxic insects
- unless symptoms are clearly characteristic of a known virus, then laboratory testing will be necessary
- with sap transmission, beware of relying exclusively on demonstration of local lesions, a bacterium may be involved.
- correlate with local weather
- plants growing in low areas of the paddock
- marginal and interveinal leaf spotting; young tender parts of plants usually affected
- distorted growth occurs when buds are injured particularly.
Diagnosing the Causes of Plant Diseases
Foliar Diseases of Cereals