Features of pathogen dissemination and epiphytotic development in agrocenoses

Some pathogens are highly specific and can infect only a limited number of host plants, sometimes even just one species. It is for them that effective dispersal mechanisms are of paramount importance. Furthermore, active dispersal allows pathogens that infect a wide range of host plants to reach epidemic levels. The rapid spread of spore-forming fungi can lead to

5 1000.: an outbreak in a very short time. E | y Indeed, to a casual observer, it sometimes seems | /| that the disease developed in one Zs | / night. Pathogens of this type sa 500 Y | 8. include Phytophthora, Botrytis cinerea, powdery mildew, and a 250 Y | downy mildew fungi, SE 2% 22“ whose airborne spores cause symp- and s symptoms of infection on most plants in the crop in a very short time. Days after first observation

Fig. 4.3. Development of an epidemic of tomato in three crops. They produce a huge number of spores and produce one generation after another.

Primary spread of a disease can occur from either one or several sources. As a result, groups of diseased plants appear, often called primary foci. In a crop, such a focus is a source of pathogen for its secondary spread. Through the soil, the disease spreads slowly and reaches epidemic levels only if there are many primary foci. Again, the tomato mosaic virus is an exception, since a small number of primary foci is sufficient for a mosaic epidemic outbreak, mainly due to the high virulence of the pathogen and its rapid transmission when working with infected plants. Fusarium wilt of carnation reaches epidemic proportions when the disease appears shortly after planting and spreads very slowly.

The dispersal mechanisms of fungi, bacteria, and viruses are very diverse. Those that are most typical for greenhouse conditions are shown in Figure 4.4 and are discussed in detail below.

Fungal dispersal

The most effective mechanism for the dispersal of phytopathogenic fungi is the transport of spores by air. Many pathogens produce - Through contamination: - Structure g by air: - Implements Spores Hands Herbicides in Clothing

‚’, OR: d u r o. R o, g NEA; | TV S M E. with plant debris

Fig. 4.4. Mechanisms of phytopathogen dispersal.

a huge number of asexual spores, each of which can initiate disease. Spore release occurs under various conditions, sometimes highly specific for the mass release of the pathogen. For example, downy mildew pathogens form spores at a very high relative humidity of the air, but the spores are released at low RH when the sporangiophores dry out and twist. Approximately the same is characteristic of Botrytis cinerea, although spore release can be facilitated by disturbing the resting infected plants (by wind or by workers walking along the rows). Powdery mildew spores are joined in long chains, and even a slight movement of air is sufficient to detach them. Sexual spores of some fungi are also air-borne, ejected by the fruiting body. For example, sclerotia of Sclerotinia sclerotiorum lie dormant in the soil, but under the influence of low temperatures for a certain period of time, they form apothecia, from which ascospores are ejected. The basidiospores of various edible fungi behave in the same way, exiting from the basidia and being carried by the air.

Once in the air, spores can travel long distances, but most often they travel only a few meters from the point of release and, naturally, remain in the greenhouse, settling on host plants, the substrate, or greenhouse structures. Through broken glass, open vents, or doors, some spores are carried outside. It would seem that outside greenhouses the chances of a pathogen finding a suitable host plant are very low; however, the number of spores is enormous, and this possibility always exists.

One of the most important ways for the spread of spores of phytopathogenic fungi infecting greenhouse crops is water. Large water droplets, upon hitting infected areas, splash, and as a result, each smaller droplet carries a spore load. Furthermore, spores are always present in water running over the soil surface in a greenhouse. Both water splashes and surface water in mushroom growing conditions are the most important mechanisms for the dispersal of fungi infecting mushrooms. Mycogone perniciosa and Verticillium fungicola spread in this way: with water splashes, the infection is spread locally around the source of the pathogen on the shelf and causes foci of infection, and with runoff water, it reaches other shelves. Soil-borne pathogens, such as Phytophthora nicotianae, which causes buckeye rot of tomato fruit if soil contaminated with spores is splashed onto the fruits of the lower clusters, also reach the susceptible host tissues with water splashes.

Some fungi spread in the soil via expanding mycelium or as a result of surface or internal infection of the root system. Pathogens that grow superficially move from one plant to another at the points of contact between rootlets. An example of this dispersal mechanism is the gradual development of vascular wilts in rows of tomato, cucumber, and especially in carnation plantings. Infection can also simultaneously result from root contact, spore dispersal, and infected plant debris on the soil surface. Causative agents of tomato and cucumber root rots are characterized by different rates of mycelial development and root colonization of their hosts. For example, re-infection of sterilized soil by the fungus Phomopsis sclerotioides, which causes black root rot in cucumber, occurs very actively, and the root system of plants can be affected quite rapidly. At the same time, Pyrenochaeta lycopersici, the causal agent of brown root rot of tomato, develops much more slowly and, even by the end of the growing season, may severely affect only half of the root system, leaving the rest untouched.

Development in the soil of rhizomorphs of the fungus Armillaria mellea is sometimes accompanied by the appearance of patches of diseased rose bushes. Old stumps remaining after the grubbing of hedges often serve as a food source for the pathogenic fungus, from which rhizomorphs spread radially.

Vectors play a dominant role in the dispersal of pathogenic viruses, but they are also of certain importance in the spread of some fungal diseases. Humans serve as important vectors of tomato stem didymella rot and cucumber black stem rot. The pathogens of both diseases form a huge number of pycnidia, and during the care of affected plants, fungal spores get onto hands and knives. It has been mentioned previously that when cutting flowers with a contaminated knife, spores can be spread from one diseased plant to thirty healthy ones. A similar situation occurs during the development of dry bubble disease of mushrooms, when Verticillium fungicola spores land on any object touched by a worker, including healthy mushrooms.

Insects are no less active as vectors of diseases. Verticillium fungicola spores easily stick to the legs of mushroom flies and are carried from crop to crop within a single facility, and sometimes from one facility to another.

The spread of spores over very long distances occurs when pathogens are dispersed with seed or infected plants. Such pathogens can be located on the seed surface (usually spores, fragments of mycelium, contaminated plant debris) or under the seed coat in the form of mycelium or spore-forming structures. A number of diseases of flowering crops are seed-borne, in particular Alternaria alternata on lobelia and Alternaria zinniae on zinnia. Primary infection foci in these cases appear as a result of the multiplication of the pathogen introduced with the seed; further spread of the diseases occurs with the help of one of the mechanisms described above.

Since many greenhouse crops are propagated vegetatively, there is always a danger of spreading pathogens with infected cuttings, transplants, and mother plants. When growing plants in regions where the natural temperature and humidity are favorable for the rapid growth of both the crop and its pathogens, the latter can be spread over very long distances, and a particular disease becomes a problem of international scale. The increase in the incidence of carnation fusarium wilt over the last decade is undoubtedly linked to the mass export of diseased cuttings from countries with a warm temperate climate. Such conditions are, in particular, favorable for the fungus Fusarium oxysporum f. sp. dianthi. To a somewhat lesser extent, the spread of Verticillium dahliae occurs with chrysanthemum cuttings, on which symptoms appear only after flower buds are set, i.e., several weeks after planting in the soil. The transport of deciduous ornamental plants is very common worldwide, and it is not surprising that introduced diseases appear on them. One of the latest examples of such international spread is the appearance of the fungus Sclerotinia rotii on Podophyllum peltatum in England.

In general, bacterial phytopathogens spread in the same way as fungi. Humans, insect vectors, water (droplets, splashes, mist), and plant material are the main pathways for bacterial dispersal. The causal agent of brown blotch of mushrooms, Pseudomonas tolaasii, is carried by water, flies, and harvesters. Pseudomonas tomato, which causes bacterial speck of tomato fruits, spreads with water splashes, as does, presumably, Corynebacterium michiganense, the causal agent of bacterial canker of tomato. The latter pathogen is also carried with seed, which leads to its spread over long distances. The causal agent of carnation bacterial wilt, Pseudomonas caryophylli, is carried with infected cut flowers (or cuttings), as is Xanthomonas pelargonii, which causes geranium wilt. With the development of national schemes for obtaining pathogen-free mother plants, bacterial wilts have become significantly less common. VIRUS DISPERSAL

Viruses do not spread through the air as individual particles; they are transmitted by vectors, plant debris, or living material. Insect vectors play an important role in the spread of many viruses in greenhouse crops, and for some pathogens, this is the only mechanism of spread, i.e., they are entirely dependent on their vectors. An example is the causal agent of tomato spotted wilt and its vector, the thrips TА рз 1аfаst. The effectiveness and speed of virus spread depend on its virulence, the number of susceptible host plants, and in some cases, the frequency of vector occurrence. One of the most striking examples of rapid spread is TMV, which is highly infectious and very easily spread by humans (on clothing and hands). It would seem surprising that such a virulent pathogen as TMV has no vectors, although aphids, whiteflies, and other insects are very common on greenhouse tomatoes without playing a role in the spread of the disease. Only a few viruses possess the same high virulence as TMV, and since most farms have adopted strict control programs for insect vectors, many of which are also pests, epiphytotics of other viral diseases break out rarely. In cases where a cucumber crop is seriously infested with Myzus persicae, the exception is infection with the cucumber mosaic virus.

Nematodes as virus vectors in greenhouses play a smaller role than in the open field, mainly due to frequent soil sterilization with steam or chemicals and the relatively short period of time during which the crop reaches maturity. Viruses transmitted by nematodes sometimes infect roses, although symptoms of the disease can be detected only on two-year-old bushes. Foci of infection develop as virus-carrying nematodes spread, which move in the soil with water and parasitize on rose roots. If the site where the greenhouse is located has a slope, the directed spread of the disease, corresponding to the slope, becomes noticeable.

Some fungi can also be virus vectors, for example, the cucumber fungus in cucumber greenhouses in Canada.

Nematodes and fungi equally contribute to the spread of viral diseases over long distances with infested soil used during plant propagation. Apparently, this is exactly what can explain the rapid spread of the viral disease of lettuce, known as big vein (peronospora losses in lettuce grown by the NFT method when using transplants taken from infected nurseries. Once developed in a planting, Olpidium brassicae forms motile zoospores and, with their help, infects each new crop secondarily after planting out transplants. The circulating nutrient solution facilitates the easy spread of virus-carrying spores.

Spread of viruses in agrocenoses

Like fungi, viruses are spread both by seed and by vegetatively propagated plant parts. Seed transmission is characteristic of a small number of viruses, of which the most dangerous are the causal agents of:

• cucumber green mottle mosaic;
• lettuce mosaic;
• tomato mosaic.

The appearance of a primary focus of infection is associated with the sowing of infected (contaminated) seed.

Role of vegetative propagation in pathogen transmission

Many vegetatively propagated ornamental crops (carnation, chrysanthemum, pelargonium) have often been sold in an infected state for many years. Such spread of pathogens acquires an international character. However, at present, in many countries, schemes have been developed and introduced for obtaining healthy starting material for maintenance propagation of plants, which have already contributed to curbing the spread of viruses and other pathogens via cuttings.