Diseases of greenhouse crops can be caused by pathogenic bacteria, fungi, viruses, and phytoplasmas. Many abiotic or non-infectious conditions mimic diseases or lead to plant disease. It is important to be able to distinguish between the two. If a disease is suspected and a pesticide is applied to remedy the situation caused by an abiotic stress, then undo expense, labor, risk has occurred.
Planning an integrated pest management (IPM) program for disease control includes sanitation techniques, monitoring techniques, and management strategies. Pesticides should only be used when monitoring reveals that they are required.
There are four basic forms of plant disease causal organisms that commonly infect greenhouse crops, fungi, bacteria, viruses, and phytoplasmas. Yet many microorganisms are beneficial. The occurrence of plant disease is a result of a complex interaction of a susceptible host plant, the presence of a pathogenic causal organism, and the environment.
Fungi
Fungi are difficult to describe because they are organisms found differing in many forms, behaviors, and life cycles. These organisms are characterized by a chitinous cell wall and filamentous growth called hyphae, which forms mycelium.
Botrytis blight or gray mold will infect most greenhouse crops and is one of the most important pathogens of above ground plant parts typically caused by Botrytis cinerea. The symptoms include brown, water-soaked spots or decay on leaves or petals.
Often confused with powdery mildew, downy mildew almost always is found in patches on the underside of foliage as soft and fluffy gray, purplish, or light brown sporulation. It is sometimes found on stems or buds. Peronospora and Plasmopara species are the primary causal fungal agents on flowering greenhouse crops.
Powdery mildews are a group of fungi including the genera Erysiphe, Leveillula, and Sphaerotheca that produce gray or white powdery growth on leaves, Figure 4. Many species are host species specific. For instance, Sphaerotheca pannosa var. rosae only infects members of the Rosaceae family.
Powdery mildews are obligate parasites existing only on living tissue and do not require plant stress or injury to infect host plants. Warm greenhouse temperatures during the day and cool at night favor powdery mildew infection. Free water is not required for spore germination. Management of powdery mildews relies on early detection, sanitation, cultural practices, environmental control, and fungicides.
Soil-borne diseases include the species Pythium, Phytophthora, Rhizoctonia, and Thielaviopsis, which collectively include the majority of the fungi that infect roots and crowns of plants. Most greenhouse crops are susceptible to one or more of these causal organisms.
Damping-off is a term that generally refers to sudden plant death in the seedling stage due to the attack of soil-borne fungi. Pythium infection is more common in wet and poorly drained soils, Figure 5. It infects young plant tissues, typically root tips, and spreads into primary roots. It causes a soft dark brown to black wet rot that disintegrates root cortical tissues, which will sluff leaving only the vascular stele.
Rhizoctonia solani is the most common species to cause root rots of greenhouse plants. Rhizoctonia infection typically results in moist, brown lesions or cankers on crowns and roots.
Thielaviopsis basicola infected roots will become very black and thickened, hence the name black root rot. The lesions will be dry compared to those found with Rhizoctonia infection.
Bacterial diseases found that infect greenhouse plants are difficult to control other than through prevention, sanitation, and removal of infected plants. Disinfectants can be used to sanitize greenhouse tools, benches and pots to provide some protection.
Viruses can be defined as non-cellular organisms. These organisms consist of nucleic acids, RNA and DNA surrounded by protein, which obligately reproduce inside host cells using the host’s metabolic machinery and ribosomes. This reproductive process forms products called virions, which protect the virus and are transferred to other cells. Plant viruses are transmitted directly through sap by contact through plant wounds with contaminated tools, hands, or by animals, mainly aphids, leafhoppers and thrips, feeding on the plant. The virus then spreads systemically throughout the plant via the vascular system.
Phytoplasmas are considered to be specialized bacteria that are obligate plant parasites found in the phloem. Aster yellows is the most significant phytoplasmas that infects greenhouse plants and is vectored by leafhoppers.
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