Exclusion means preventing the entrance and establishment of pathogens in uninfested crops in a particular area.
It means using certified seed or marijuana plants, sorting bulbs before planting, discarding any that are doubtful, possibly treating seeds, tubers or corms before they are planted, and most especially, refusing obviously diseased specimens from dealers. For example, tare soil returned to trucks at sugar beet dump stations should never be returned to production fields because of contamination by nematode and rhizomania diseases from other infested fields.
In order to prevent the import and spread of plant pathogens into the country or individual states, certain federal and state laws regulate the conditions under which certain crops may be grown and distributed between states and countries. Such regulatory control is applied by means of quarantines, inspections of plants in the field or warehouse, and occasionally by voluntary or compulsory eradication of certain host plants.
Plant quarantines are carried out by experienced inspectors, stationed in all points of entry into the country, to stop persons or produce likely to introduce new pathogens. Similar quarantine regulations govern the interstate, and even the intrastate, sale of nursery stock, tubers, bulbs, seeds, and other propagative organs, especially of certain crops, such as potatoes and fruit trees.
For example, a Michigan quarantine prohibits the entry of seed potatoes produced in regions infested with rhizomania disease of sugar beet unless accompanied by a certificate indicating the production field has tested free of the disease.
Some Common Methods for Isolating Bacteria
Another common method for isolating bacteria from infected leaves as well as other plant parts is to cut several small sections, five to 10 millimeter squares, from the margin of an infected lesion so as to contain both diseased and healthy-looking tissue.
These are placed in a surface sterilant solution, making sure that the surface is wet, and after about 15 to 30 seconds the sections are taken out aseptically one by one at regular 10 to 15_second intervals, so that each of them has been sterilized for different times. The sections are then blotted dry on clean, sterile paper towels or are washed in three changes of sterile water, and finally placed on the nutrient medium, usually three to five per dish.
Those sections surface-sterilized the shortest time usually contain contaminants along with the pathogen, while those surface-sterilized the longest produce no growth at all because all organisms have been killed by the surface sterilant. Some of the sections left in the surface sterilant for intermediate periods of time, however, will allow only the pathogen to grow in culture in pure colonies. These colonies of bacteria are then subcultured aseptically for further study.
Diagnosis of Major Disease-causing Pathogens
Many marijuana plant diseases are recognized by characteristic symptoms on host plants. More specific identification of disease may require observations of characteristics of the causative pathogen. Fungi are identified by their spores and fructifications (fruiting bodies), also called spore-bearing structures.
These are examined under the compound microscope directly after removal from the specimen. The specimen should be kept moist for a few days to promote fructification development, or the fungus may be isolated and grown on artificial nutrient media, and identification made on the basis of fructification produced on the media.
The shape, size, color, and manner of arrangement of spores on the fruiting bodies, as well as the shape, color, orientation, etc. of the fruiting bodies, are sufficient characteristics to suggest, to one somewhat experienced in taxonomy of fungi, the identity of the particular fungus pathogen.
Bacteria can be isolated from leaf spots and blights by surface sterilizing the area to be cut with sodium hypochlorite (Clorox), removing a small part of the infected tissue with a sterile scalpel, and placing it in a sterile plate containing a nutrient medium.
Protection is the use of some protective barrier between the susceptible part of the suspect or host and the pathogen.
In most cases this is a protective spray or dust applied to the plant in advance of the arrival of the fungus spores; sometimes it means killing insects or other inoculating agents; sometimes it means the erection of a windbreak or other mechanical barrier.
Fungicidal sprays that act as protectants are used to control Cercospora leaf spot of sugar beet, especially in those fields where inoculum has carried over from the previous year. The principle of protective fungicides is to disrupt the natural sequence of infection. These fungicides act on the leaf surface to kill the newly germinated spores. Flowable sulfur is used as a protectant fungicide to control powdery mildew of sugar beet.
There is a long list of chemicals available in the literature that can be used in present-day protective spraying and dusting, along with eradicant chemicals. The commercially sold chemicals are provided with instructions or notes on compatibility and possibilities of injury.
A commercial grower can do his marijuana plants irreparable harm instead of the good he intends if he doesn’t follow the instructions supplied. Spraying is never to be undertaken lightly or thoughtlessly. Read all of the fine print on the label; be sure of the dosage and the safety of that particular chemical on the plant species to be protected.
Bacterial Wilt DryBean
Investing in Healthy Marijuana plants look good, grow well, and are productive. Plants remain healthy as long as conditions favor normal plant growth and development. Sometimes plants are unhealthy, and this occurs when something irritates the plant. The irritation may be somewhat continuous, acting over an extended period, or it may occur nearly instantaneously. Continuous irritation causes disease; instantaneous irritation causes injury.
Disease frequently is expressed by production of symptoms. Some common types of symptoms on specific parts of plants include rotting, stunting or swelling of roots; cankering, rotting, discoloration, distortion, elongation, or stunting of stems; wilting, spotting, blighting, rusting, mottling, discoloration, distortion, or stunting of leaves; and spotting, blighting, stunting, discoloration, distortion, or mottling of fruit.
Disease-resistant and tolerant varieties are the cheapest, easiest, and most efficient way to reduce disease losses.
Varieties should be selected that possess resistance or tolerance to one or more disease organisms. For some diseases, such as the soilborne vascular wilts and the viruses, the use of resistant varieties is the only means of ensuring control.
Certified marijuana seed of resistant varieties is available and sold commercially. The use of varieties of plants resistant to particular diseases has proved to be very effective, i.e., stem rust of wheat, rust of dry bean, and Rhizoctonia root rot of sugar beet. Most plant breeding is done for the development of varieties that produce greater yields of better quality.
When such varieties become available, they are then tested for resistance against some of the most important pathogens present in the area where the variety is developed and where it is expected to be cultivated. If the variety is resistant to these pathogens for that area, it may be released to the growers for immediate production.
If, however, it is susceptible to one or more of these pathogens, the variety is usually discarded, or sometimes it is released for production if the pathogen can be controlled by other means, e.g., chemical, but more often it is subjected to further breeding in an attempt to incorporate into the variety genes that would make it resistant to pathogens without changing any of its desirable characteristics.
There are degrees of resistance to certain diseases, some varieties being completely immune, others partially susceptible. Resistant varieties may become susceptible to new races of a pathogen, i.e., dry bean varieties Beryl and Olathe were resistant to rust races present at the time of their release, but are now susceptible to new rust races.
Eradication involves the elimination of a pathogen once it has become established on a plant or in a field
It can be accomplished by removal of diseased plants, or parts, as in roguing to control virus diseases or cutting off a cankered tree limb; by cultivating to keep down weed hosts and deep ploughing or spalding to bury diseased plant debris; by rotation of susceptible with nonsusceptible crops to starve out the pathogen; and by disinfection, usually by chemicals, sometimes by heat treatment.
Spraying or dusting marijuana foliage with sulfur after mildew mycelium is present is eradication, and so is treating the soil with chloropicrin to kill nematodes and fungi. Soil treatment with various nematicides (Telone II, Temik 15G, Counter 15 and 20G) is useful to control sugar beet nematodes.
Tan spot, caused by the fungus Pyrenophora tritici repentis, is a major leaf spot disease of winter wheat in the Great Plains of North America. It has become an increasing problem in wheat cropping systems using conservation tillage. This disease can be managed by applying a three-year conservation tillage rotation system called ecoflow.
Eco Fallow is defined as crop rotation system of controlling weeds and conserving soil moisture with minimum disturbance of crop residue. In this system, corn or sorghum is seeded directly into winter wheat stubble in a winter wheat-grain sorghum/corn-fallow rotation.
The uniqueness of this system is that one crop is planted directly into the residue of a different crop rather than into the residue of the same crop. This crop rotation-fallow system effectively breaks disease cycles, such as tan spot, which involve pathogens that survive in crop residue.
General Principles of Managing a Disease
General management considerations: the practical reason for studying marijuana crop diseases is
to develop economical measures for control.
Controls must be based on knowledge of the specific disease, pathogen life cycles, the time and the method of infection, the plant parts affected, the method of causal agent dissemination, and certain other agronomic
and economic considerations.