Corn Diseases

Corn Diseases


(Music) My name is Emmanuel Byamukama, and I’m the Extension Plant Pathologist and I will present on corn diseases and later Connie Strunk will present on soybean diseases. But one other person that I’d like to introduce to you is Kay Rudin and she’s the Research Associate and she really does a great job when it comes to fungicide trials both on soybean, corn, and wheat. So for diseases on corn, I would like to ask you what are some of the diseases that you’re seeing out there. Any issues with Common Rust? How about Goss’s Wilt? Some, okay well so the the corn diseases part is the last four pages of the handout. So if you can go there and what I would like to start with sort of the highlight general information. This applies to any crop, any pathogen, is what plant pathologists call disease triangle okay, and what that means is you have to have three factors for disease to develop. This it sounds like common knowledge, but you’d be surprised how many of us consult or try to think about disease triangle before we go out and apply a fungicide. Okay, so for disease to develop you need three factors; the host, which has to be susceptible. You have to have a virulent pathogen, so not any pathogen you can cause disease. You may have a race that’s not virulent on a particular cultivar. So a susceptible host, virulent pathogen and a favorable environment. And the reason I wanted to highlight the three factors is because of the environment that we’re having. This is really unusual for August weather in the threat of the cooler, and so because of that we may not see some common diseases that develop on corn and soybean okay. And talking about cooler conditions, we may not see Grey Leaf Spot develop because Grey Leaf Spot will not develop when temperatures are below 75 degrees, so it’s a warm living pathogen okay. So again highlighting the importance of disease triangle when it comes to managing diseases. Ada mentioned applying fungicides, you know kind of in a way that is not following whether it is not disease to warrant a fungicide application. I’ve had cause you know people asking I have Common Rust on the lower leaves should I apply a fungicide and ask how much is out there, okay. That would be a a question of we’ll discuss later. But disease triangle is important. When it comes to a hybrid you want to ask yourself what hybrid I plant. Is it something that is acceptable to Goss’s Wilt. If I have Goss’s Wilt now, next time in my field, I want make sure that I have that cultivar that is resistant to the Goss’s Wilt. Well if it is Common Rust, whatever it is you want to keep track of what you see on your field and then the selection of the cultivar or hybrid will plant for next season. So disease triangle is really important. Both the environment, the host, and the pathogen must interact together and the the extent of the overlapping of these three factors will determine how much this is the pressure we have for a respective season okay. All right now, let’s look at some of the diseases. We’ve seen developing on corn, and the first one is Goss’s Wilt. I do have here very nice looking leaves that have really good symptoms of Goss’s Wilt. The way to diagnose Goss’s Wilt is look for those black freckles on the leaves that would be diagnostic feature for Goss’s Wilt. Sometimes we also see what is soaking especially at the end of the lesion where it’s expanding on the leaf. And then you can also see a shiny ooze of the bacteria, okay. Now the bacteria that causes the wilting and the leaf blight survives on corn residue and to enter the plants through wounds. And that’s why we’ve seen most of the reports of Goss’s Wilt coming from the northern counties where they’ve had a number of hail events. So hail, high winds, insect damage, those will be avenues for the bacteria to enter the plants. Now if you have a plant infected so early in the season, then you may have the wilting stage like the whole plant will get gets wilted. If you get infection later in the season, then you may have the leaf blight phase of this disease. So how do we manage Goss’s Wilt? Should we apply a fungicide? Why not? Because fungicides only control fungal pathogens, okay and we’ve seen in other crops if you apply a fungicide, you may make some other diseases even worse, okay. So it’s important you diagnose what you have so that you can have proper treatment or a proper management decision for that particular problem. So Goss’s Wilt can be managed through different ways and they are what I’ve called three R’s. So rotation, in other words plant and none host for the pathogen, so for Goss’s Wilt, plant rotating into bins is going to reduce inoculum because the bacteria survives on infected residue, okay The second R is plant a resistant hybrid. We have likely the companies provide ratings for Goss’s Wilt and so if you’re planting corn on corn or you know you’re having no-till corn, then the innoculum is going to be high so you may want to look at using a resistant hybrid of corn. The third R is the residue management because the pathogen survives in the residue. So if you’re in a part of the country where tillage is is okay, then tillage would reduce the inoculum, but in other areas you know where tillage is not recommended, then you can use other methods to control Goss’s Wilt. Right now the second bacterial disease that we have seen on a very low incidence is a Holcus Spot. Holcus Spot is caused by another bacteria called Pseudomonas syringae pv. syringae. This is a bacterial disease that has a wide host range. It can also occur on on soybeans, but likely on corn, this disease never develops never develops levels are to cause concern. In other words, we don’t have high disease pressure that causes yield loss, okay. The bacteria also like Goss’s Wilt survives on corn residue, so it’s management is going to be effective if you rotate away as well as managing the source of inoculum, which is the residue, so tillage again can reduce the source of inoculum for this disease Holcus Spot. The way to diagnose it is if you look on the second page of the con diseases, it’s look at the round kind of elliptical shaped lesions that are, there color is between white and tan to white color in the center of this lesion okay. They are really small so you can’t confuse them with Goss’s Wiltl and again this disease is not a big concern. Most of the hybrids have good resistance to Holcus Spot, but occasionally you may come across this disease. The other disease that we see is Common Rust and just like the name suggests, a Common Rust is pretty common. Every field that I’ve gone to I have found this disease. Every cornfield pretty much is going to have Common Rust. Now unlike the rest of the diseases on corn, rust pathogen does not survive in South Dakota. Spores have to be blown from southern states and northwards so crop residue or rotation is not going to do anything about rust development, and this is where I’m having questions. So how much rust do I need to see before I can apply fungicide for managing Common Rusts. So any of us has already applied a fungicide on corn. That’s it. That’s really good because in here we have different fungicide timings that we have for demonstration purposes as well as experimentals that variable rating but as you you can see this is an untreated corn, and it’s pretty much nothing. So just like Ada mentioned, we need to be careful when it comes to applying these management tools that we have because we may misuse or overuse some of these and in future where we’re going, we may land ourselves into trouble. So we’re really encouraging scouting and making sure that you have enough disease that’s developing to warrant a fungicide application. That way you’re not being too hard on natural enemies, as well as setting a stage for resistance development. Okay so Common Rust. We’ll recommend about 15% of the entire plant covered with the pustules to be the threshold for fungicide application. Less than that, the rust is not going to be a significant factor to cause yield loss, okay. Most of the rust now that is developing on corn is on the lower leaves, and we know that the leaves that impact yield would be from the ear leaf upwards. Okay, so if you see some rust on low leaves it’s not a big deal. I would wait before I can try a fungicide. The other disease that we’re seeing is Common Smut. I gave an ear here of corn that has nice-looking galls of Common Smut. Common Smut this season was a concern because it was appearing on leaves as well, so growers went wondering what is this the it’s coming through leaves? But Common Smut can affect any part of the plant. As long as it finds the entry point to their part of the plant. So for leaves this season where we had held some of those developed Common Smut. Now this fungus those galls you see a full of spores, okay, and then they would dry out and spread by wind or all over the place and they can survive in soil for a long time. Okay, but likely the for Common Smut, it’s not the disease that will cause yield loss and so it will infect a few plants here And there. It never develops to a levels that will be will cause yield loss. Okay, so we shouldn’t apply a fungicide to control Smut because it rarely develops to become a disease of concern. Okay. The other disease that you may see here, and there is a Physoderma Brown Spot. This is a fungal disease that also survives on corn residue and it usually loves a lot of moisture, so where you see it develop would be on the leaf nodes, so where the leaf joins the stem. Because sometimes we have moisture sitting there and when the spores land there they infect that part of the plant and later when the leaf expands, then you will see kind of these lesions at this at the center or the midrib of the leaf, and the picture I have in the handout should show you the symptoms of this disease. So it’s also a residue born and so controlling it would mean rotation and and residue management as well as using resistant cultivars. This disease likely also never drops to levels that will cause yield loss. So it’s usually not a concern, but it’s something that we have noted in cornfields so far this season. Some other diseases that may develop, but we have not seen include Gray Leaf Spot, Eye Spot, Northern Leaf Blight. This morning. I saw one lesion on just one plant here in this field. All these three diseases survived also on corn residue, but they love warm conditions as well as high humidity. We have had enough humidity to see some of these diseases but because the temperatures are more to the cooler side, we may have a delayed development of some of these diseases. But anytime you have residue borne disease, you know you have the inoculum in the soil. So the best thing to do is to scout and make sure that it is a significant level of the disease that warrants fungicide application, okay. So scouting and then rotation you know this would be for the next growing season of course, we already have a crop here and then resistance. Then if you have to apply a fungicide, then it should be guided by how much disease you have there. Again consulting the three factors what hybrid you have, whether the weather conditions do we have conditions that may promote disease development, or should we you know watch the situation keep scouting and to have enough disease pressure to are on fungicide application. So with scouting, you can save yourself fungicide application and increase the profitability if the disease pressure is not that high to warrant fungicide application. So any questions on diseases on corn? Okay, no issues. Okay great, so I’ll hand over to Connie, and then she’ll talk about soybean diseases. (Music)

Leave a Reply

Leave a Reply

Your email address will not be published. Required fields are marked *