As temperatures warm-up, our planting season take up and watching plant development as progresses is a fundamental tool to control diseases. Texas A&M AgriLife Extension information states that plant diseases can occur at any stage during the course of plant growth. The rapid, accurate diagnosis of the cause of a disease, along with the implementation of a rapid treatment, is essential to ensure the protection of the crop. Certain infectious diseases caused by living, microscopic organisms have the potential to rapidly ruin a crop. However, for any particular vegetable, these diseases are not that numerous and, so, it would not be difficult for a grower to become familiar with them and take proper preventative action. Diseases caused by nonliving things (i.e. not infectious) can be much more difficult to diagnose. Usually, it is easier to rule out an infectious agent as the cause of a disease before investigating possible nonliving (abiotic) causes. This stresses the need for the grower to become familiar with the more common infectious diseases that can occur on the crop. This chapter provides an overview of the science of diagnosis and treatment of vegetable diseases. The grower is advised to consult other references listed at the end of the chapter for more detailed information related to a particular crop. Disease is the outcome of an interaction between the host, the disease agent, and their environment. If the cause of infectious disease, the pathogen, is next to the host, nothing will happen unless environmental factors are favorable for its infection and development within the plant. With foliar pathogens, there is usually a minimal period of leaf wetness required to stimulate spore germination and infection. For some soilborne pathogens, infection occurs in combination with high soil moisture and certain critical soil temperatures. Knowledge of conducive environmental factors for the more important vegetable diseases presents an opportunity for more effective management: the disease can prevented by altering some of the environmental factors, or, when such factors cannot be altered, steps can be taken to minimize the impact (e.g. fungicides could be applied in advance of a period of sustained rain which would favor foliar diseases. Not all diseases are caused by pathogenic organisms. Determining whether a disease is caused by a pathogen, or has nonliving (abiotic) causes requires not only the examination of individual plants, but also, noting the pattern of symptom occurrence in a field. Examine individual plants for unusual symptoms, such as leaf spots, wilts, stunting, fruit rots, misshapen leaves, cankers and stem blight. Roots should be examined for galls, root rot and necrosis (dead areas). Fields should be observed to determine if the problem is widespread and whether different plants species in and around the field are affected, which could indicate an abiotic cause. Symptoms with a nutritional or physiological cause have a more widespread occurrence within a field than infectious diseases. Initially, most disease causing pathogens will be isolated in areas and spread outward from those areas. Also, weeds or nonrelated crops are not typically affected. Soilborne pathogens are even more restricted within a field than foliar pathogens. Fungi are multicellular microscopic organisms that can grow to their food, usually in the form of filamentous strands. Their growth pattern is radial, so on surfaces such as plant leaves, the effects of their growth may be seen as circular spots. However, fungal infections of other plant parts, such as roots, may produce no visible structures. Some symptoms can indicate these infections. For example, browning of the water-conducting tissues of the stem, in combination with wilt, can indicate infection by the Fusarium wilt fungus. Other disease symptoms, such as blight (a general death of tissue), which can have a variety of causes, may require laboratory testing to confirm fungi as a cause. Fungi can produce specialized structures, such as spores, which are used for reproduction, dissemination through space and time, and survival. Sclerotia are structures that function in the long term survival of many soilborne pathogens. The southern blight fungus which infects many vegetables forms sclerotia resembling mustard seeds. Most fungi that infect leaves require free moisture to initiate infection, with the exception of powdery mildew fungi, which need only high humidity to initiate infection. Because our current weather conditions, with high level of rain, fungi infections are expected. Bacteria are single celled microscopic organisms, which survive by becoming dormant. The most notable exception to this is the pathogen that causes common scab on potato: this is a filamentous, multicelled bacterium that produces spores. They can be transported by wind driven rain, by insects, or the movement of infected plant parts, including seed. Bacteria that infect leaves may cause circular spots, but irregular shaped lesions that don’t extend beyond veins are more characteristic for bacterial infections. They can also cause soft rots of vegetative parts that are usually characterized by a foul smell. Other bacterial diseases that produce symptoms such as wilt require laboratory analysis for diagnosis. Mycoplasmas are bacteria-like, only structurally simpler and smaller than bacteria. They are transmitted by leafhoppers. The most notable disease of vegetables caused by a mycloplasma is aster yellows, which affects carrots, celery and related plants. The symptoms of aster yellows are distinctive: leaves have a bronzed appearance and flowers are abnormal (leaf-like tissue grows from them, instead of petals.) For more information on this or any other agricultural topic please call the Hopkins County Extension Office at 903-885-3443.
Coming Up: Basic Home Vegetable Gardening, Professional Ag Workers Building, 957 Connally Street, Sulphur Springs, Texas, March 31, 2015 starting at 6:30 PM. Registration $10. Homeowner Maintenance of Aerobic Treatment Units, April 17, 2015 at the Professional Ag Workers Building located at 957 Connally Street in Sulphur Springs. Registration: $100. You must preregister for this training by calling 903-885-3443.