Most of us never think where the food that lands on our tables come from. We enjoy the rices, various other starches, beans and and even the garments that we wear daily. We don’t think about where the cotton that makes the shirt we’re absolutely in love with starts out. All we know is that the end product is necessary for us to live our lives in one way or another.
There is so much time and effort that goes into planting and harvesting crops like rice, cassava and even cotton. It’s labour intensive and a long process. Farming is a multi layered activity. It all starts out with planting the crops, cultivating them and finally harvesting them. In between all those processes, things can go wrong. One of the most devastating ways is leaf blight.
If you’re not a farmer, you can’t rightly be expected to know what leaf blight is and how it may affect someone's livelihood. It’s a common enough problem that much information can be found on it, but I’ll try to condense for you and give the bullet points. We’re going to look at how it affects the most prominent crops the world over, rice, cassava and cotton. We’ll look at;
- What is bacterial leaf blight and what causes it?
- How does it present in rice, cotton and cassava?
- How is different or similar in other plants?
- What does blight mean for a season of crops and how is it treated?
- Can you prevent the disease from occurring and what is the most effective?
Bacterial Leaf Blight is a condition that all plants can suffer. It’s caused by a virus known as Xanthomonas oryzae. The mechanics of how it happens are multi-layered and there is no real way to avoid it, but it can be controlled and reversed. Without getting too technical about it, it’s a form of plant disease that is not only waterborne, once it has infected your crops it then also becomes seedborne and you end up planting already blighted crops.
Some of the crops that are the most high risk for bacterial leaf blight are cotton and cassava, but most especially rice because of the way the disease is transmitted more easily through water. Although we will be talking about all these of plants, it’s more easily illustrative at first to look at rice to get an idea of the path of the disease.
- The disease is introduced to plants through natural openings such as growth cracks on roots
- The water pores that take water to different parts of the plants.
- If the plant already has rot spots or leaf wounds, it will use these openings as well.
- It will then grow, infecting the plants veins and causing a blockage of the the xylem. The xylem is one of two systems of a plant that takes water throughout the plant.
- Once this is blocked the plant won’t be able to get the water that it needs to sustain it and will become dry, damaged and will eventually wilt and die.
When rice is harvested, stems, roots and even seeds are left behind. Because the bacteria lives on dead plants, it will move through the rice paddy in the waters in its irrigation system, storms and other rainwater. It can also ooze out of any opening in the plant, including the lesions the disease creates and be carried to other nearby plants on the wind.
Now you may be asking yourself where does this disease come from though? It doesn’t just float up out of the ether, no. The bacteria usually starts and is incubated in wild grasses, or any grass, before it’s transmitted on the wind to nearby crops. Because of this it’s very hard to predict when or where leaf blight will occur, although some regions like asia are more prone.
The Disease shows up differently in the specific plants it affects. In some crops it’s more easily spotted than in others.
Cassava bacterial blight shows up on the leaves of the plant as;
- Water soaked spots that can appear anywhere from green to blue.
- Once the spot starts to rot, they will then look brown.
- They will be transparent when held up to the light.
- Sometimes the spots will be circled by green rings of chlorophyll.
Once the leaves turn transparent they become easily dissolvable in water, so that even a light dew will destroy the leaves. When dried they’ll resemble something of a thin scale like entity. Only very rarely does the blight make it to the roots of the plant, but when it does it will make them very dry and brittle, crumbling to the touch.
Additionally the leaves of the plant will turn from a healthy green to a yellowish green within a day or two, gradually shedding. The leaves will look almost as if they’ve been burnt. In high humidity environments, the spread is accelerated and plants will die faster because the humidity will cause it to spread to roots, stems and other areas it wouldn’t in a drier environ.
One of the most common ways to blight on crops is through water soaked spots, or lesions. It’s no different for bacterial blight of cotton. You’ll first notice the disease creep into your crops through the appearance of these slightly angular looking, light to dark brown spots. They’ll usually be surrounded by a circle as well, just in this case yellow rather than green.
The blight can spread to the leaf veins. These will look like tiny lightning bolts, the stem and root of the plant may be affected as well causing significant damage. In the most extreme cases it also extends to the boll of cotton inside, presenting as water soaked depressions. This often results in unintentional defoliation that leads to a significant loss of crops.
Unfortunately cotton blight is perhaps the most difficult to salvage once the it has started. Mainly because the infection starts in the seed plant and lives on the fuzzy hairs that coat the seed. Blight can start in the middle of the crop life cycle or as little as fourteen days before the harvest cycle begins. This unpredictability is what leads to crop death.
Perhaps the most well known well documented forms, is that of bacterial blight of rice. This is likely because rice is such a well known commodity that is consumed all over the world by so many people.
Like the previous types of blight we discussed, bacterial blight of rice attacks the leafy parts of the plant. It then moves on to the stalks, seeds and roots. Because rice cultivation is so very heavily dependent on water, the blight spreading in this instance is almost a certainty. When this happens entire crop yields are often lost.
So what does the blight look like in rice? Rice blight, like cotton blight most often starts in the seedling stage, although wind and water can transfer the virus to already healthy plants. So unknown to you, it may already be present at the start of your planting cycle without you even knowing it. You’ll know you have rice blight if;
- Your seedlings start to turn a greyish-green colour.
- They’ll start to roll up on themselves.
- Leaves will start to turn yellow and have a straw-like look as the disease progresses.
- At the later stage, there will be water soaked spot on the leaves of the plant
- These spots will turn a yellowish-brown eventually causing the entire leaf to turn yellow, wither and die.
The blight will be slightly different in all plants. The virus that attacks the plants, Xanthomonas, alters its genetic structure slightly depending on the plant that it’s affecting so that it will be more effective. So for instance, Xanthomonas axonopodis, or Citrus Canker causes leaves and fruit to drop prematurely. Entire groves are likely to be destroyed in an effort to halt the disease.
Unlike rice, cassava or cotton, citrus canker often spells death for an entire crop of citrus fruits. Likewise Late Blight, a disease that affects tomatoes and potatoes late during their planting cycle often causes loss of crops because it’s a particularly strong strain. It also has spores that can spread to nearby plants ruining those as well.
All of these diseases present similarly and you can usually tell that they are blighted because of the water soaked brown, grey or yellow spots on the leaves and fruit of the plant.
Just because your crops have become blighted, doesn’t mean that you have to say goodbye to a whole season of planting and raising. While for some forms of blight this may indeed be the case, for crops such as rice, cassava and cotton if caught early enough it can be reversed or salvaged. You may not get a whole crop yield from planting, but you’ll have saved something.
This is perhaps the most easy type of blight to treat and manage, because cassava is not generally cultivated in a humidity and moisture rich environment. This cuts down on just how fast and far the infection can spread. Since the disease rarely spreads to the root of the plant there are many way to minimise spreading and to save the crops already planted.
- The first thing to do is choose the cuttings that you’ll be planting carefully by inspecting for signs of blight. If you’re unable to do this, use a source that you trust when you obtain your cuttings.
- If using cuttings from an old planting or plants that are already in ground, choose the parts of the plant that are the oldest and growing at least one metre above the planting soil.
- Don’t plant close, next to or downwind of an infected crop as the wind may carry disease organisms to the new plantings on the rain.
- Do not plant in the same areas every season. The planting site should be rotated and one site should only be used within a one to two year span.
During growth if notice signs of blight;
- If only a relative few plants seem to be infected, cut those plants out and leave the rest.
- Be sure to do this only when plants have not yet been watered or you risk spreading the disease to uninfected plants via the water.
- Thoroughly clean all tools used to remove infected cuttings so as not to spread the disease to uninfected plants.
- Burn or bury all infected stems that you’ve collected during the harvesting process.
Because of how blight in cotton usually presents which is first on the seedling, there really are only a few ways that it can be treated, and they all start with making sure that the seedling being prepared for planting is pathogen free.
One of the few ways that seems to have worked well in the last few years has been a switch to heavily resistant varieties of cotton. A few other solutions that have emerged are;
- Soil rotation. Because this particular strain of the the virus can not exist outside of soil with crop residue, it’s relatively easy to ensure that crop blight doesn’t carry over to the next planting season.
- Harvesting a field as soon as blight is detected even if it’s before the start of the harvest season.
- Use seeds that have been delinted. Delinted seeds have had the hairy parts of the their seed covering removed leaving a smooth seed.
- Delinting, particularly acid delinting cuts down on the the chance of pathogens such as blight or boll worms being on the seeds that you plant. It also reduces seed rate which leads to more vigorously resisting crops.
This is an easier form of blight to treat, mostly due to there is so much data in regards to treating different types and symptoms because we’ve been doing for such a long time. There are now some blight resistant forms of rice as well, which have become a game changer. But for the non-resistant forms, there are some steps that can be taken;
- This goes without saying, but making sure that you’re using pathogen free plantings is the first step in avoiding blight.
- Using a good balance of rich nutrients, while avoiding excess nitrogen.
- Ensuring that water irrigation and drainage channels are clean and clear.
- Keep fields clean by removing weeds, ratoons and unwanted seedlings.
- Plow under stubble and seedlings.
- Allow fallow fields to completely dry to minimise the risk of disease surviving and transferring to flowering fields.
- Try to use disease resistant varieties of rice when possible.
- Introduce a targeted solution before damage to help induce resistance in varieties that are not naturally resistant.
These are all things that you can do before and during the initial phase of planting and flowering. Most prevention is done through the planting of disease resistant varieties as well as application of specific chemical solutions to help boost the resistance of plant to blight
Unfortunately blight is not something can be wholly prevented. There is no foolproof method outside of diseases resistant varieties of ensuring that whatever we plant remains blight free. As much as we may try to control the environments in which we plant, we have no control over the weather. One seedling is all it will take to wreak havoc on an entire seasons worth of planting.
That’s not to say that we shouldn’t take all the preventions that we can and watch carefully for the first signs of blight. We’ve already spoken of some of the best ways to prevent blight. But those are just stemming a potential problem. The only real way to ensure crop survival alongside your preventatives is to plant disease resistant varieties of your crops.
I hope that this discussion on bacterial leaf blight has given you a better understanding of how the disease works from start to finish. How to spot the first signs of the disease and how to prevent it before it starts or how to reverse if at all possible. Now that we’ve learned all of this together today, as don’t hesitate to pass along your newfound knowledge to a friend.