IDENTIFICATION AND CONTROL FOR MANGO BACTERIAL BLACK SPOT DISEASE

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.

Seedlings control

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.

The fungicides

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).

Fig.15: The mango tree is treated by the systemic fungicide that enters the tree (in blue), but it doesn’t prevent BBS from further spreading (in red)

The spraying schedule

The spraying schedule depends on the kind of fungicide in use, so read the instructions of the product carefully. To find out what dosage is needed for your specific purpose, it is recommended to consult with fellow farmers or any other knowledgeable person. 

Usually the treatment of BBS is additional to other fungal diseases like anthracnose.  From first flushes to post-flowering you can keep up the usual spraying routine, just as you do to control anthracnose. But after the trees have lost their flowers and if BBS is present, you need to intensify spraying.

Here is a general spraying routine against Anthracnose and BBS:

1.  When first flushes appear – use contact fungicides (copper)

2.  Shortly before flowering use systemic fungicides

3.  During flowering, DO NOT APPLY ANY FUNGICIDE AT ALL, otherwise you damage the flower and kill useful pollinating insects

 4.  After flowering, apply contact fungicides 

5.  From this time on: continue to spray every two weeks: start with contact fungicides (copper) and two weeks later use systemicfungicides. Then copper and then systemic again. Then, continue to apply different types of contact fungicides (copper) in three week intervals. Note: the dates on this figure are an example; exact dates depend on your local conditions!

6.  STOP SPRAYING THREE WEEKS BEFORE HARVEST 

So under this spray regime, you would apply one copper and one systemic spray before flowering and up to 5 copper and 2 systemic sprays after flowering. DON’T USE MORE THAN THREE SYSTEMIC SPRAYS IN A YEAR. 

NOTE:  Be aware of rain when you plan your spraying activities. Rain washes down the chemicals and you have to repeat the treatment.

Rotate between different kinds of contact and different kinds of systemic fungicides: it is important that each type contains different active ingredients in order to avoid the development of resistances from the disease (because when it happens, the disease will not be affected by the fungicide anymore, and so you are wasting your money).

Working together is the best solution

As mentioned before, BBS is a disease you cannot fight alone: try to work with farmers around your farm, find out with them where the disease is present and where it is originating from and then, jointly confront the disease from all sides. This will save time, money and work for everyone!

Source: http://teca.fao.org