Black Rot is a common name given to a variety of diseases caused by various fungal or bacterial pathogens and water moulds (oomycetes). The symptoms vary depending on the pathogen type and host plant.
This article looks at the most common types of Black Rot diseases that affect edible and ornamental plants, including Black Rot of cruciferous vegetables, Black Core Rot of Mandarins, and Black Rot of Orchids, as well as the exotic disease Black Rot of Grapes.
Xanthomonas campestris pv. campestris
Black Rot of cruciferous (Brassicaceae) vegetables such as Broccoli, Cabbage, Cauliflower, and Kale, is caused by a bacterial pathogen. It infects the leaves, causing them to yellow and eventually take on a scorched appearance. This bacterial disease also causes the leaf veins to blacken, hence the name of the disease.
Infected leaves have light brown to yellow V-shaped lesions, typically starting along the leaf margins. The centre of the lesion becomes dry and brown and as it continues to spread on the leaf, gives the entire leaf a scorched appearance.
As the disease progresses, the entire plant can be affected, with leaf veins, stems and roots turning black.
There are currently no products available to treat Black Rot of cruciferous vegetables. The best way to help protect your plants is to practice good garden hygiene.
The bacteria spreads with water splash from rain or irrigation and enters plants through natural openings, damage caused by insect feeding, other pathogens, or mechanical damage. As it spreads with flowing water, it's best to avoid overhead watering when possible. It can also be spread from infected seeds collected from diseased plants, so it's important to use disease-free seeds or seedlings.
The bacteria can also survive on weeds and overwintering crops or debris left in the garden. Remove diseased plant material and bin to prevent spread. Rotating cruciferous crops and not growing them in the same place each year will also help to reduce the incidence of Black Rot.
Phytophthora spp., Pythium spp.
Black Rot of Orchids can be caused by a variety of fungal or bacterial pathogens or water moulds. But the most common cause of Black Rot of Orchids is likely to be from infection of water moulds, such as Phytophthora or Pythium. While they are both water moulds, they attack different parts of the plant and as a result, the symptoms are different too.
Phytophthora
Phytophthora infects the stems, leaves and roots of orchids. It causes irregular-shaped, water-soaked spots that start dark brown before becoming black. The infection typically starts with spots on leaves before spreading down the stem, but it can also start on the stem and work its way into the leaf.
Pythium
Pythium infects the roots, rhizomes or pseudobulbs, causing them to become blackened and soft. It's primarily a below-ground disease but blackened lesions may appear on leaves and stems.
These water moulds spread with water splash, so avoid overhead watering. Quarantine any infected plants and use secateurs to help remove any affected plant parts, ensuring you sterilise the secateurs between cuts (dip secateurs in 1-part bleach to 10 parts water to sterilise).
Phytophthora
To control Black Rot caused by Phytophthora, treat with Yates Anti Rot Phosacid Systemic Fungicide. Certain orchids may react to the fungicide, so it's best to test on a small area and wait a few days to observe for any adverse effects. If no adverse reactions are observed (e.g., leaves dropping or yellowing), then you can continue with a more widespread application.
Pythium
Black Rot of Orchids caused by Pythium is typically a result of poor-draining growing media. It's best to repot the plant into a fresh mix. Remove the plant from its pot, prune away dead and affected roots, then repot into a fresh, well-draining mix like Yates Specialty Potting Mix Orchids. Also, ensure the pot has enough drainage, with multiple holes in the base of the pot.
Alternaria alternata
Black Core Rot of Mandarins is a common disease that affects mandarin fruits. It's a fungal disease that infects the inside or core of the fruit, causing it to blacken and rot. The infection can be difficult to detect as the fruit often looks healthy on the outside, but once it's open, the rot is revealed.
Affected fruit may have a sunken, brown to black lesion on the base or 'stylar' of the fruit. However, this is not always present, even after infection has taken hold. Infected fruit will have a blackened internal core, which spreads and causes the fruit to rot. As the disease matures, fluffy, grey fruiting bodies appear on all the affected parts.
There are no chemical treatments currently available to control Black Core Rot of Mandarins. The fungus enters through wounds or natural openings, which can be hard to avoid. Mealybug infestations can exacerbate problems with Black Core Rot of Mandarins, so if you spot them, it's best to treat them as soon as possible. Spray mealybugs thoroughly with Yates Nature's Way Citrus & Ornamental Spray.
Bin affected and mummified fruit and do not compost any fallen fruit unless they have been checked and cleared for signs of infection.
The best way to help reduce the likelihood of infection is to keep your mandarin tree healthy. Promote healthy plants by feeding regularly with Yates Dynamic Lifter Fruit & Citrus Plant food Pellets and control any pests at first sight. Click here for more information on How to Grow and Care for Mandarin.
Phyllosticta ampelicida syn. Guignardia bidwellii
Black Rot of Grapes is a serious disease that can affect all above-ground parts of the Grapevine. The infection is typically prevalent in spring after extended periods of rain and can result in significant crop losses. At present, the disease is considered exotic in Australia, so if suspected, call the Exotic Plant Pest Hotline for further information (1800 084 881). All infected material, including canes, shoots, and fruit, should be bagged and isolated.
Small, circular lesions appear on the leaves in spring. They are brown with reddish margins and as they enlarge (to a max diameter of 7 mm), they develop black interveinal margins and grey-tan or red-brown centres. The spore-bearing structures of the pathogen, also known as pycnidia, appear in the centre of the lesions and are visible as small, black dots.
On new shoots, stems and tendrils, the lesions are tan before becoming sunken and purple-black. They may be elliptical or elongated. Pycnidia may be present and as the canes grow, the bark may split.
On fruit, small, pale dots (1 mm) appear and are surrounded by a widening brown halo. If left untreated, the brown lesions continue to expand until the fruit becomes completely covered and begins to rot. Berries shrivel and turn dark brown with various pycnidia developing on the surface. Ultimately, the fruit dry and shrivel, turning into hard blue-black mummies that may fall or remain on the vine.
The symptoms look very similar to symptoms caused by other diseases, such as Black Spot or Anthracnose (Elsinoe ampelina), Alternaria spp., Botrytis cineraria, or Phomopsis Cane and Leaf Spot (Diaporthe ampelina). However, the presence of pycnidia in the lesions is usually a tell-tale sign of Phyllosticta ampelicida, but a DNA sequence analysis can also be used to confirm the presence of the pathogen.
There are currently no fungicides or treatments available for use against Black Rot of Grapes in the home garden.
After reading this information and if you now believe the disease on your Grapevines to be Black Spot or Phomopsis Cane and Leaf Spot, you can treat vines for these diseases with Yates Mancozeb Plus Garden Fungicide and Miticide.
If you notice that the bottom of your tomatoes are blackened and soft, it is very likely they are suffering from Blossom End Rot. This is a physiological disease related to issues with calcium deficiency and/or irregular watering. It is not caused by a fungus and can be corrected by addressing soil nutrient deficiency and watering issues. Click here to read more about Blossom End Rot.
Blossom End Rot Prevention in Your Garden
Blossom End Rot Prevention in Your Garden
