Occurs when the pathogen comes in contact with the plant.
The actual organisms which come in contact with the plant are called the "inoculum". Any part of the
pathogen which can attack the plant is called "inoculum".
If the inoculum lays dormant over winter then infects the plant in spring, this is called the primary
inoculum, and it is causing a primary infection.
Inoculum produced from this infection is caused secondary inoculum and can cause secondary
infection of the plant.
Inoculum may be present in the soil or in dead plant material near to the plant being affected; or it may be brought in to the area with seed, new plants, soil, on the wheels of a car, on boots or shoes, or
even carried by the wind. Inoculum can survive on weeds or infected plants nearby, and move onto
cultivated plants when conditions are favourable.
Pathogens move into plants by breaking through the plant surface, by entering through wounds, or through natural openings (such as stomata).
Some fungi only penetrate through one of these methods.
Bacteria mainly enter through wounds.
Viruses and some micro-organisms (micro-plasmas and some bacteria) enter through wounds made by vectors (NB: A vector is a disease carrier... Aphis carry viruses, they inject their mouthpiece into the plant creating a wound and placing the virus inside the plant).
Nematodes normally enter through direct penetration.
This is the process by which the pathogen establishes contact with the cells or tissues which it is going to affect. In this stage the pathogen grows and invades parts of the plant which it will infect.
Changes to the plant can be either obvious or obscure at this point. You might see discolouration or necrosis as the disease moves through the plant OR it may be that the changes are microscopic and necrosis or other symptoms are not seen until the next stage (Growth and reproduction).
4. Growth and Reproduction
The pathogen now grows and develops within the part of the plant which it inhabits.
It then begins to reproduce itself.
Spores or new organisms produced in the growth and reproduction stage are moved to other places where they can sooner or later infect a new plant.
Mostly this dissemination is carried out by agents such as wind, water, insects, animals or man.
The main symptom of these diseases appear as either:
Dead (dark coloured) spots.
Sunken spots with a slightly raised rim.
Spots can be small or large. Dark brown, purplish, red or black sunken spots appear on stems, fruits or seeds. This disease may cause fruit drop and fruit rot.
- Common diseases on beans, grape vine and willows.
- Also deciduous trees including walnut, oak, sycamore, linden, elm and hickory.
- Various ornamentals including azalea, cyclamen, sweet pea, poinsettia and violet.
- Some fruits including apple, cranberry and raspberry.
- Spray with copper-based sprays such as Bordeaux mixture.
- Use a proprietary fungicide.
Anthracnose of Beans (Colletotrichum lindemuthianum)
Symptoms on small plants are seen as dark brown marks around 12mm long on the stems. On mature plants spots appear on the stems, leaves and pods, veins become blackened, black sunken spots to 12mm across may appear on the pods, and long black sunken marks may occur on the stems. On a commercial scale this disease is considered relatively rare now due to widespread use of approved disease free seed.
- Plant only disease-free seeds - do not collect seed from diseased plants.
- Spray with a protectant fungicide, ensuring undersides of leaves are sprayed.
- Do not plant beans in an area previously affected by this disease for a minimum of two years.
Anthracnose of Grapes (Glomerella sp.)
Also called ripe rot because it appears when the fruit is almost ripe. It is most likely to occur in warm, humid conditions. Symptoms on white grapes are small red brown spots appearing at first before spreading and becoming darker when they are dark purplish in the centre and light brown on the edge. On red grapes the berry rots the same as for white grapes, but there is no real change in skin colour. This disease can also affect other fruits.
- The fungus spreads from affected stems, leaves and fruit left over from the previous autumn. This makes it important to collect and burn clippings, fallen leaves, fruit, and so forth which might spread the infection. It can also be spread by wind or insects.
- Pay attention to plants growing between or under vines. Rotate under vine plantings.
- Spray with a copper based spray or protectant fungicide.
Anthracnose of Cucurbits (Colletotrichum orbiculare)
This is a very destructive disease on many cucurbits, but particularly watermelon and cantaloupe. All parts except roots can be affected. Symptoms are firstly small, water-soaked, yellow patches or spots on the leaves which grow up to 2cm in diameter. They turn black on watermelons and brown on other cucurbits. Fruit stalks sometimes develop dead patches, which cause fruit to become darkened, then shrivelled, and then to die. Fruits may become infected close to ripening (round, sunken, watery patches develop on the skin, then expand and develop dark centres). Stems can also sometimes become infected, leading to death of the entire plant. Infected parts become dry and break up. The disease can overwinter in fruit or even in seed.
- Spray with a protectant fungicide every 7 to 14 days once plants start to run.
This problem is seen as small, often brown, dimples on the skin of apples. Underneath the pits, the cells appear dried-out. It seems to occur on apples that have a nutrient deficiency, particularly calcium, although erratic weather conditions can also be involved.
- This may be achieved by applying water and nutrients on an even, regular basis.
- Spraying with calcium nitrate (50g/10 litres water) in early summer may also be beneficial.
This is quick death of plant parts. Parts of a plant (e.g. leaves) simply stop growing and die, often without rotting (though rotting may occur later). A blight is generally sudden, and as well as death of parts of a plant can sometimes involve leaf or fruit drop.
Blights are often named according to the part of the plant they infect and the plant variety they occur on e.g. azalea petal blight is a blight where dead tissue develops on the petals of azalea flowers.
- Hygiene is most important for all blights - remove and burn all infected tissue as soon as detected.
- Copper-based sprays will help control most blights, though some require other specific chemical sprays.
There are different varieties of this disease which cause both blights and leaf spots. Sometimes the leaf spot and blight effects appear together. Plants affected include: cucurbits, carnations, carrots, potato, tomato, violet, zinnia, umbrella tree (Scleffera), ivy, aralia.
- Copper-based sprays or proprietary fungicides.
Bacterial Blight (Pseudomonas syringae)
On mulberry, the disease (Pseudomonas syringae pv. mori) first affects young leaves as they burst from the bud. This causes blackened dead edges to the leaf resulting in leaf distortion. The surrounding area is yellowed.
- Remove infected leaves and spray with a copper based fungicide.
On Peas, the disease (Pseudomonas syringae pv. pisi) is common in cool to warm, damp conditions, entering the plant through damaged tissue or stomatas. Damaged parts first appear to be water-soaked, but then change to dark brown. On pods the damage is usually kept to the edges.
- Destroy infected plants.
- Use disease-free seed.
- Rotate crop on a three year cycle.
Fire Blight (Apple & Pear)
Fire blight is a bacterial disease causing fruit to die soon after petal-fall, and affected leaves turn brown and remain on the tree. There are strict quarantine controls on fruit imported from places known to have fire blight.
Halo Blight (Beans)
On leaves, the bacterium (Pseudomonas phaseolicola) causes small angular brown spots surrounded with a lime halo. Wilting and stunting of leaves may occur. On stems, elongated, dark green water-soaked areas occur. On pods, circular spots with greasy centres appear. Cool showery weather increases the risk of infestation.
- Destroy diseased crops immediately after harvest
- Use only disease-free seed
- Reduce traffic between infected crops and non-infected areas.
Leaves may develop spots or patches of dead tissue. The leaves may drop, or may hold on the plant. The dead areas may spread to other parts of the plant, and might become larger. A number of different fungi can cause leaf blight. A large variety of different types of plants can be affected.
- Spray with copper-based sprays when leaf buds are half open, and again when leaves are half grown.
Petals discolour and rot, resulting in a slimy brown mass. In time, this mass dries out and becomes papery and brown. They remain connected to the plant unlike healthy flowers that normally fade and then fall off. If the humid weather continues, black seed-like bodies up to 4mm wide called "sclerotia" may develop. These can cause future infection in the following seasons. This disease should not be confused with grey mould where petals may only rot, without the presence of the sclerotia. Failure to correctly identify the fungus can result in unsuccessful fungicidal applications. This fungus is most common on rhododendrons, primarily on Kurume and Indica azaleas. It may also infect kalmias.
On Eucalypts, the disease (Ramularia pitereka) is prevalent in cool, shady, moist environments with poor air circulation. Small spots (brown with a thin purplish-brown margin) develop on one edge of leaves causing distortion. Leaves frequently appear shiny white. Sunken brown areas may develop on the stems and petioles.
- Use copper-based sprays.
- Improve environmental conditions such as ventilation.
On pines, the disease (Diplodia pinea) causes brown necrotic areas at the ends of branches. Attack is more common on weakened trees.