Dr. Tom Isakeit, Professor and Extension Plant Pathologist
There is intense interest in growing hemp (Cannabis sativus) commercially in Texas. Along with considerations of applying the best agronomic approaches for growing it, there is a need to consider potential pitfalls, particularly insect pests and diseases. There is very little information on hemp diseases in Texas. The national host indices list just three fungal diseases of hemp in Texas, but this probably reflects a lack of observations. With a small hemp acreage in Texas, there will probably be a “honeymoon” period of little or no disease, but as acreage increases, especially if hemp is grown under monoculture, diseases will become noticeable and might affect yield and quality.
I will address the potential disease problems of hemp grown in Texas and this assessment is somewhat speculative, but is informed by my experience of seeing diseases on many crops throughout Texas, as well as a visit to commercial and university hemp fields in Kentucky in September, 2019. The fields in Kentucky were nearing harvest or being harvested and given that the Kentucky climate is wetter than Texas, I was thinking that this would be a “worst-case” scenario for what could be encountered in Texas. Ironically, my visit coincided with twenty-five consecutive days without rain in Kentucky, a condition that is quite common in many (most?) areas of Texas.
In my opinion, the most serious disease facing hemp in Texas is cotton root rot (“Texas root rot”) (Figure 1), a disease which is absent in Kentucky, and most of North America, for that matter.
This is a soilborne disease, caused by the fungus, Phymatotrichopsis omnivora, and it is one of the reported diseases of hemp in Texas. The good news is that although this pathogen is widespread in Texas, it is not everywhere. It is absent from the High Plains and East Texas. Growers already know whether the pathogen is present in their fields or not, if they have repeatedly grown susceptible crops such as cotton or alfalfa. They should not grow hemp on spots in fields where this pathogen occurs. The hemp will die.
There are many fungi that cause foliar diseases on hemp. The severity of these diseases depend upon many hours of sustained leaf wetness, caused by rain or dew. Such foliar diseases will not be as much of a problem in Texas as they are in states like Kentucky. The area of East Texas (east of Interstate 45) will likely have the greatest risk for chronic foliar disease development. However, foliar fungal diseases may be a problem in other areas in years with above-average rainfall.
My most interesting observation during the Kentucky tour was seeing hemp leaf spot (Figure 2), which was quite widespread in Kentucky. This disease has also been seen on hemp in at least ten other states. It has only been recognized in the past few years, following widespread planting of hemp. The fungus that causes it, Bipolaris/Drechslera gigantea, has a wide host range, mostly grass species. It was reported in Texas on bermudagrass more than a century ago, and more recently, we documented it as a pathogen of barley in Burleson county. It struck me as being a “stealth” fungus during the Kentucky tour. It suddenly appeared throughout fields, even though the weather had been dry for several weeks. It was not evident whether there was any yield loss from this disease, but Dr. Nicole Gauthier, hemp pathologist in Kentucky, reported up to 100% yield loss in other, wetter years. This pathogen may well make an appearance on hemp grown in Texas, but yield loss will probably be a function of how wet the growing conditions are, as well as the relative susceptibility of the variety.
One foliar disease caused by a fungus, powdery mildew (Figure 3), is likely to occur in Texas, but the potential for yield loss is not yet known. There are at least two species that occur on hemp. The species, Golovinomyces spadiceus, documented on hemp in Kentucky, is widespread in the United States and also infects sunflower and okra.
Overall, soilborne pathogens may be more prevalent on hemp in Texas than foliar diseases, but the presence of these pathogens can be known from the occurrences on other crops that are susceptible to them. As mentioned previously, cotton root rot is the number one pathogen threat, in fields where that pathogen occurs. Hemp is susceptible to Verticllium wilt, but this fungus does not cause disease throughout Texas. It has been a problem on cotton and other crops in the High Plains and Far-West Texas. Hemp is susceptible to charcoal rot, caused by Macrophomina phaseolina . This fungus is widespread throughout Texas and has a wide host range, but only causes symptoms on drought-stressed plants. Hemp is susceptible to the root knot nematode, which is a problem on cotton in the High Plains, but this nematode also occurs throughout the state on other crops as well. I have not been able to find any reference to the reniform nematode as a problem on hemp in the literature. The lack of information may mean that no one has tested for susceptibility or no one has noticed a problem. The reniform nematode has a wide host range of dicot plants and is problem on cotton in several growing areas of Texas. The soilborne fungus, Sclerotium rolfsii, which causes southern blight, has a wide host range that includes hemp (Figure 4). Infection by this fungus is favored when soil is moist or wet and soil temperatures are high. This pathogen was previously reported on hemp in Texas.
Figure 4. Southern blight (Sclerotium rolfsii) on hemp. Left: Fungal growth on crown. Right: Close-up of survival/infective structures (sclerotia) that resemble mustard seeds. Photo: T. Isakeit
There are minor soilborne pathogens that can cause disease on stressed hemp. One cause of stress is poor rooting of hemp cuttings (Figure 5). Cracks in the stems of cuttings can also allow the entry of the fungus, Botyrtis cinerea, which can cause stem canker.