Managing Rots in plants

A rot is a disease which causes decay or disintegration of plant tissue.
The decay may be either dry (where the tissue remains hard), or wet (where the tissue becomes soft and squashy).
It may affect only one part of the plant (eg. fruit or roots) or the whole plant.
Rots can be caused by either fungi or bacteria.

Control: Chemical sprays can be used to prevent rots on fruits and vegetables. Damaged plant parts can be removed and destroyed.


A bacterial disease causing stem rot in potatoes.
Symptoms: Black coloured rot in potato extending from the point where the underground stem joins the root.
The variety Sebago is very susceptible to this disease.

Control: Maintain good drainage and don't over-irrigate. Plant complete tubers and minimise damage or wounding to tubers. The spread of the disease is facilitated by insects, so good insect control is important. Dusting tubers with an insecticide can be effective.


BLACK ROT (Xanthomonas campestris)

Also called "bacterial leaf scald" in crucifers (ie. cabbages, cauliflowers, broccoli)

Leaves develop small tan greasy spots with yellow halos, between the veins which later unit and become brittle and drop out giving the leaf a tattered look.It usually starts at the margins of the leaves producing a V-shaped damaged region.

Control: use seed treated with hot water (50 degrees C for 30 minutes), plant in soil free from un-decomposed plant residues, prevent overcrowding of seedlings and plant recommended cultivars. Ploughing in of old crop is fine provided plant material is thoroughly decomposed before planting future crucifers. Remove weeds and other plants nearby which allow the bacteria to survive from one crop to another (ie. host weeds like shepherd's purse and wild radish and even ornamental stocks).



This is a fungal disease found on the bottom (or blossom) end of a tomato fruit.

Symptoms: A dark, flattened or sunken leathery type rot, normally due to irregular patterns of watering, such as excess rainfall followed by a period of drought. A contributing factor is a deficiency of calcium. If left unattended the rot will spread to the rest of the fruit.

Control: Add calcium oxide to the growing media or spray the plant with a 1% calcium chloride solution. Good deep soil preparation, good nutrition and mulching to conserve moisture are all beneficial. Infected fruits should be removed and destroyed.



This is common on seedlings and vegetables. Brown patches appear on the stem near soil level. The stem may be completely ring-barked.

Control: Soil drench with Quintozene. Dust annual and vegetable seeds with a fungicide prior to planting. Ensure good drainage and ventilation, and be careful to minimise damage to the stems of seedlings, particularly during transplanting.

On Citrus: Symptoms include gum oozing from the bark, wet patches of bark, or bark becoming dry and flaking off. It occurs close to soil level on the trunk.

Control: Cut away infected parts with a sharp knife and paint with a copper-based paste (eg. Bordeaux or Kocide).


CROWN ROT (Sclerotium rolfsii)

Also known as Southern Blight in America. Affects almost all plants except some field grain crops. Seedlings and vegetables are particularly affected.

White growths appear on the stem, particularly in wet weather, and can spread over the soil. This fungal growth produces droplets of oxalic acid which kills tissue which comes in contact with it.

Control: Remove diseased plants and treat the soil with Terraclor, or Fongarid.



This develops mushroom like fruiting bodies, usually yellow to brown in colour, that appear above the soil, usually near the base of trees, or old tree stumps, or growing directly out of decomposing wood, such as old logs. It attacks the roots of woody ornamental plants & some herbaceous species (including Strawberries). It can spread extensively, with some examples known to cover hectares in area in some moist, shaded forested areas.

Control: For trees that are in the early stages of infestation, exposing roots to air for a diameter of 1 metre around the tree can reduce the effects of the rot. Removing and burning badly attacked plants is vital, followed by soil fumigation.


STEM ROT (Pythium sp)

Pelargoniums prone to this disease exhibit the base of the stem blackening and withering causing the whole plant to wilt and die. The fungus is moved in water and soil and is more likely if the soil is wet.

Control: destroy infected plants. Drench soil with Fongarid. Provide new disease free plants and potting media in a well drained pot or site.



Some bulbs, corms, tubers & rhizomes tend to rot during their dormant (ie. non-growing period). The likelihood of rotting is decreased if they are removed from the soil, cleaned and stored dry until the next growing season when they are replanted. Even bulbs stored this way can sometimes rot, particularly if they get too wet (in humid climates; poorly ventilated places; or stored too close together).

Basal Rot -where the base of the bulb rots.

Tulip Fire -fungal growths occur on outer scales of tulip (flowers & shoots can be damaged).

Narcissus Smoulder -growths appear on outer scales of Narcissus.

Corm Dry Rot -Black spots appear on corms, these grow, merge & the whole corm rots.

Gladioli Core Rot -the centre core of gladioli develops soft rot which then spreads

Control: Dipping, or dusting in fungicide such as Copper dust or Benlate (benomyl) as soon as they are dug. Remove and destroy infected bulbs immediately (ideally burn them).



Research by an American scientist, Dr Alex Shigo, has demonstrated that trees have a natural mechanism to resist decay caused by harmful microorganisms.

The tree is made up of a series of "compartments". When microorganisms first attack a tree, chemicals are deposited around the wound, and in effect, form a barrier which prevents invasion behind the barrier. However, some microorganisms can grow through this barrier, allowing other microorganisms to move in behind the first invaders. A snowball effect occurs, with successions of microorganisms causing further damage, and making the spread of infection difficult to contain.

Another "compartment" which acts to prevent the spread of decay is the "wall" of new wood and bark tissue which is produced each year. This new layer envelopes the old tree, thus helping to wall off infections.

Should the tree rot be caused by the secondary infection of a disease, such as brackets, seek professional help from an arboriculturist. Trees with rots may be dangerous with the possibility of limbs or the whole tree being blown over in strong winds.



Sclerotinia rot is a fungus disease which affects a wide variety of plants. It is particularly bad in late summer/early autumn on lettuce and beans. Infection can occur on any part of the plant above ground level, followed soon after by the appearance of soft brownish rot. Affected tissue becomes covered with whitish mould. As the disease develops, black pebble-like fruiting bodies are formed on the white growth.

On lettuce and cauliflower, infection usually occurs through injuries to bases of stems or to lower leaves. The leaves wilt and soon after the whole plant collapses. With cauliflowers going to seed, the fungus attacks the flower stalks causing flowers to wilt and die.

Late summer and autumn bean crops appear most susceptible in the post flowering stages...infection often occurring through petals which fall onto healthy stems and leaves.

When the fungus infects a spot on the stem, the part of the plant above that spot will wilt and die quickly. This is very serious if the attack occurs at ground level.

CONTROL: Space plants wider apart. This creates a better air circulation and reduces humidity. Spray with Benomyl every 10-14 days as soon as any sign of the disease is observed.



Citrus Brown Rot (Phytopthora sp.)

Fruit show a grey-brown mark that grows larger and darker.

Control: Keep branches pruned so that they do not hang near the ground, where they brush against the soil, or get splashed by soil particles. Spray with copper-based sprays, such as Kocide or Copper oxychloride.

Brown Rot of Stonefruit (Sclerotinia fructicola)

Common on ornamental and fruiting Prunus species, particularly under humid conditions, during flowering and fruit ripening periods.

Symptoms: The first symptoms are brown spots on the flowers, which spread rapidly, becoming a furry mass which is greyish brown in colour, and later shrivelling and drying up. These may remain clinging to the tree for long periods. Twigs bearing the flowers may develop small, sunken brown cankers which can spread, causing blight in the stem.

Fruits start appearing to be affected as they near maturity. They go brown in patches, which spreads rapidly. Yellowish mould often appears in concentric circles. The fruit becomes completely rotted, and then shrivels up. It then either hangs from the tree or falls to the ground, where it provides a source of infection for the following year.

Control: Remove twigs with cankers, or infected flowers as early as possible. Infected twigs can be removed during pruning. Remove infected fruit as soon as they are located and burn them. Harvested fruit should be handled with care to reduce wounding that would allow infection. Spray with Bordeaux when flowers show the first sign of pink colouring, then regularly with Benomyl or Zineb. Control of insects will also reduce the likelihood of infection through insect wounding of the fruit.


APPLE FRUIT ROT (Trichoderma harzianum)

Symptoms: Firm, tan-brown spots occur. They may develop into moist lesions. They have a characteristic musty "coconut" odour. This disease is most common on pome fruit, such as apples and pears. Trichoderma spp. are common in soils. Harvest and post-harvest treatments appear to be the main problem sites. Soil contamination of fruit, or of dipping bins in which the fruit was dipped, combined with the presence of wounds leads to infection.

Control: Benlate gives some control. Cleaning soil off fruit and maintaining good hygiene in dipping containers is very important.

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