Growing mango is full of challenges and farmers should know about them to succeed in the mango production. The more farmers know, the better they will succeed. This short film (and its description) is dealing with a disease called Bacterial Black Spot (BBS). In 2010 it was first identified in West Africa and is now a major threat for mango growers.
It weakens branches and causes fruit drop. If not treated, it can devastate an entire plantation. Once the disease breaks out in a certain area, it spreads further and further every year. This video (and the description below which summarize the video) aims to provide some advices on how to recognize the disease and stop it.
1. Identifying BBS
The ‘Bacterial Black Spot’ (or BBS) disease is named after the black spot marks, which develop first on leaves and then spread on fruits (Fig.1).
Fig.1: BBS disease (black spots marks) on leaf (left) and on fruit (right)
Be careful not to mix up BBS with another disease called “Anthracnose”. Both appear similar at early stages but if you look close enough at the black spots (sometimes even holes), the difference can be noticed (Fig.2).
Fig.2: Black spots of BBS (left) and of Anthracnose (right)
Around the black spot or holes of BBS, there is a yellow halo, which is rough, not smooth (Fig.3). At times, the leaves will crumble/fall apart.
Fig.3: Black spot of BBS with its yellow halo
Whereas in anthracnose, there is no yellow halo around the black spot (Fig.4).
Fig.4: Black spots of anthracnose WITHOUT yellow halo
Apart from the leaves, the BBS also shows on branches and wounds like in the Fig.5 can appear on young twigs. On older branches BBS looks drier, like in the Fig.6. Anthracnose would not produce anything similar to these symptoms.
Fig.5: BBS wounds on young twigs (young branches)
Fig.6: BBS wounds on older branches
On the next stage of the BBS development, some cankers can appear on the branches or leaves and substance (similar to water) is oozing from it.
2. Sources of infections: how the infection spreads and what can be done to prevent the spread?
The spread of the infection
BBS is a bacterial disease, highly specific to mango (it cannot affect other crops), and it often comes from infected seedlings. BBS spreads in the orchard when it is wet and stormy, so the risks of infection are higher during the rainy season. BBS bacteria use damage points caused by storms to enter trees (Fig.7).
Fig.7: Broken points (branches, trunk) where BBS bacteria can enter
Strong winds disperse the disease and it can be spread about 100 to 200 meters each year (Fig.8). This way BBS can move from a neighboring plantation to another. This means that it is not enough to control BBS only in 1 single farm: as long as the disease remains in an area, it will spread through the plantations and even if one farmer gets rid of BBS, there is a high probability that it comes back again from the farms around which failed to treat the disease. The only chance to actively get rid of BBS is to work together with all neighbors: farmers should join hands and agree on a coordinated control plan.
Fig.8: Two neighboring mango plantations (Farm A/Farm B) with some infected trees on farm A in 2016 which will affect the entire plantation of the farm AND the plantation of the neighbor farm B in 2017
Windbreak trees solution
A solution is to plant wind breaks trees on the side of the plantation that receive the most of the wind (Fig.9). At these strategic points they serve as a shield against the wind (it slows it down) and so, reduce the level of infection. You can find further technologies on wind breaks on TECA: Soil conservation through multi-purpose wind breaks/shelter belts in Vietnam
Fig.9: Wind break trees on the windward side of the farm B plantation. The wind break is planted between the two plantations
The Acacia tree is one good option for a windbreak. Once it grows up, it develops a dense structure, which slows down the speed of wind. This reduces damage caused by storms and also possibilities of BBS to spread onto the neighboring field.
Often BBS is introduced to an orchard via infected seedlings/seeds/scions brought in from nurseries. When getting new trees from a nursery (Fig.10), make sure the place is free of BBS. It is also advisable to find out where the owner of the nursery harvested the scion: try to get as much information as possible and make sure, not to import the disease to a healthy area right from the beginning.
Fig.10: Nursery of mango tree seedlings
Sanitary control of transport vehicles and tools
Another opportunity for BBS to spread is immediately after harvest because diseased fruits and leaves might end up in transport vehicles. Trucks now become a fast moving threat to healthy areas along the way: try not to carry infected fruits through the mango plantation. The BBS-bacteria can get onto your working tools, even on dry days. After working in infected areas, the tools should always be disinfected – use methylated spirits or strong alcohol for the disinfection (Fig.11).
Fig.11: Desinfect the tools with methylated spirits or strong alcohol after using the tools
Diseased wood control
The greatest source of infection, however, is diseased wood in the farm. The disease hides in cankers (Fig.12), and once infected, the canker and all other diseased wood serve as sources of further infection. The greatest weapon against BBS is to thoroughly remove all this material.
Fig.12: Cankers on a branch which could host the BBS disease
3. Manual BBS control
To control the disease, first, it is necessary to get a clear picture of which parts of the farm are infected. The orchard should be inspected at least once a month in dry season; in rainy season you have to intensify the scouting. During harvesting and pruning workers are in close contact with trees, so this is an opportunity to inspect the upper canopy. Always try to remove all infected material immediately.
Removing all infected material
The control of BBS aims to protect first the flowers and then the fruits, that is why the flowering period (and just after) is a crucial time for BBS control. Removing infected wood at this time will greatly reduce the chances of infection.
As mentioned before, BBS also uses cankers (usually found on older branches) to hide. Check the trees carefully and remove infected branches. Apart from the wood, you also have to get rid of infected fruits: check with precaution the immature fruits and get immediately rid of fruits which show early signs of infection.
BBS manual control is hard work but if you manage to remove all infected wood from your orchard in one season, you will experience a great relief in the next season.
NOTE: Never work in the orchard when it is raining or when the canopy is still wet. Since BBS spreads well under wet conditions, you could accidentally help the disease to spread and so do more harm than good.
Burning the infected wood
After removal the infected material (wood, cankered branches, infected flowers or fruits, etc.) should be burned (Fig.13). Don’t let any diseased wood lay around on the farm: the infection spreads from cut-off material just like from leaves and branches still on the tree.
Fig.13: The infected wood has to be burned
Any infected material is a source of further spreading of BBS. You should get rid of it rigorously and as soon as possible.
4. Chemical control
Spaying fungicides is an additional protection against BBS but chemicals ALONE cannot solve all the problems. A good prevention system (trying not to introduce the disease in the farm) and mechanical control (removing of the infected material) are the major priority! Without these, the investment in chemicals is a waste of money.
Always wear protective clothing (overall, rubber gloves, rubber boots goggles, nose mask, etc.) when mixing chemicals and spraying them as they could be harmful for human health even in the long term.
There are two different types of fungicides: contact and systemic fungicides.
Contact fungicides (or copper) are often copper based. They stay on the surface of the tree and protect only parts covered with spray. Rain can wash them off! (Fig.14). Contact fungicides provide a protection that prevents new BBS infection and so that slows down the spread of BBS.
Note that copper fungicides could be harmful for bees and soil. Use it as less as possible, and only if necessary.
Fig.14: Contact fungicides (or copper) acting like a protecting cover (in blue) against infection of BBS (in red). After the rain (picture on the right), the fungicide is washed away and the BBS can infect the tree
Systemic fungicides enter inside the tree and distribute into its different parts – trunk, branches and leaves. This also means that fungicides enter into your fruit! Systemic fungicides heal the tree from the inside BUT do not stop BBS from further spreading (Fig.15).