MICROBIOLOGY Study Guide - 2012

CHAPTER 15. FOOD MICROBIOLOGY

15.12. MICROBIOLOGY OF FRESH FRUITS AND VEGETABLES

The surfaces of fruits and vegetables naturally harbor various microorganisms, the majority of which do not cause spoilage or disease. These microorganisms constitute the epiphytic microflora of plants. The primary Representatives of the epiphytic microflora on fruits include Yeasts, lactic acid Bacteria, acetic acid bacteria, and bacterial and fungal spores. ROOT vegetables typically carry numerous soil-dwelling microorganisms on their surfaces. The microbial load on healthy fruits and vegetables varies within a wide range, typically from 10⁴ to 10⁶ per 1 cm².

Fruits and vegetables possess defensive systems against the penetration and proliferation of microorganisms. First, the cuticle serves as the primary barrier preventing microbes from invading the juicy Tissues of fruits and vegetables. Second, plant Cells themselves contain a wide array of protective compounds: organic acids (including benzoic and salicylic acids, which exhibit aseptic properties), anthocyanins, Flavonoids, Essential Oils, Tannins, phytoncides, etc. Third, living plant Organs can actively respond to the invasion of phytopathogenic microorganisms by synthesizing specific antimicrobial compounds known as phytoalexins.

During storage, fruits and vegetables remain viable, sustaining various physiological processes (such as Respiration and Transpiration—the Prevention of Water loss). Damage to plant produce incurred during harvesting, transportation, and storage creates pathways for the entry and development of various agents responsible for fruit and vegetable spoilage.

The Development of storage diseases depends primarily on the Temperature and humidity within storage facilities, as well as the Methods of packaging and stacking the produce. The principal causative agents of fruit and vegetable diseases are mycelial Fungi, and to a lesser extent, bacteria and yeasts. This is because fruits and vegetables contain high levels of CARBOHYDRATES and negligible amounts of Proteins, while many also maintain a low juice pH. As fungi proliferate, the concentration of organic acids in the fruits decreases, the juice pH rises, and conditions become favorable for bacterial growth. In vegetables with a near-neutral pH, bacterial spoilage is more prevalent than in fruits and berries.

Despite having a natural defense system, the resistance of berries and fruits is limited, particularly when they are ripe. Damaged and overripe fruits are the first to undergo spoilage.

In practice, depending on the external symptoms of the disease or the causative agent, several common forms of spoilage are distinguished, most frequently referred to as rots.

Late blight or brown rot is the most common disease affecting potatoes, tomatoes, and other solanaceous crops. The causative agent is the fungus Phytophtora infestans. Brown spots appear on the leaves of infected plants, which later become covered with a white mycelial bloom. Affected leaves and haulms fall to the soil. Fungal zoospores reach potato tubers while still in the field (during harvesting). The surface of an infected tuber develops irregularly shaped spots that are firm to the Touch, featuring a dark brown color with a lead-gray sheen. In cross-section, the affected tissue appears brownish, spreading from the site of infection toward the center of the tuber. During storage, potatoes infected with late blight become susceptible to other saprophytic fungi and bacteria that exacerbate the spoilage process: tubers turn dry and crumbly or wet and rotting. Resting forms of the late blight pathogen (zygotes, chlamydospores) can overwinter in plant debris within the soil.

Storing vegetables at low temperatures and low air humidity, combined with good ventilation in the storage facility, helps prevent the spread of the disease.

Fusarium dry rot is one of the most common potato diseases during winter storage. The disease is caused by fungi of the genus Fusarium: F. solani, F. sambucinum, F. coeruleum. Potato tubers become infected both in the field and in storage facilities. Fungal-infected tissue becomes dry, and the Skin turns wrinkled. The surface of the tubers becomes covered with a whitish bloom containing abundant crescent-shaped conidia. With continued storage, the tuber shrivels, shrinks, and hardens. Voids filled with fungal mycelium form inside the tuber. The development of dry rot is facilitated by tuber damage, chilling, and infestations by slugs and other pests.

Fusarium bulb rot of onions is caused by the species Fusarium серае. Affected bulb scales turn brown and soften, and the surface of the bulb becomes covered with a white bloom. The infection originates at the basal plate of the onion, which is why the disease is commonly known as "basal rot." The disease spreads very rapidly in storage. The temperature range for Fusarium growth spans from 0 to 35 °C. Lowering the storage temperature to 2–4 °C reduces the spread of the disease.

Potato wart. The CAUSATIVE AGENT OF this disease is the fungus Synchytrium endobioticum. The source of infection consists of cysts that, upon germination, produce uniflagellate zoospores that penetrate potato tubers. Initially, small, smooth, and light-colored bumps form on the tuber surface near the eyes, which subsequently develop into wart-like growths filled with resting fungal spores. In severe infections, the entire tuber becomes covered with bumps and loses its market value. These growths gradually disintegrate, turning into a foul-smelling slimy mass.

Potatoes affected by wart disease are unsuitable for storage and Processing. During rainy summers, particularly at temperatures around 20 °C, about 50 % of the potato crop can be destroyed in the field by wart disease, with the remainder perishing post-harvest. The primary cause of the disease is soil contamination with fungal spores, which can persist in the soil for decades.

Common scab is a widespread potato disease occurring in crops grown on light sandy, sandy loam, and calcareous soils. The causative agents are actinomycetes of the species Streptomyces Scabies, S. tricolor, S. cretaceus. Streptomycetes are soil-dwelling microorganisms that inhabit, overwinter, and contaminate tubers in the soil. The tuber surface develops rounded, flat, or slightly raised star-like cracked pustules or ulcers. Affected tubers quickly wilt. Parasitic fungi and bacteria penetrate through these cracks, causing the potatoes to rot.

Powdery scab. The causative agent of this disease is the fungus Spongospora subterranea (Class Chytridiomycetes). It spreads when potatoes are cultivated in heavy clay, loam, and peat soils. The disease manifests as light-colored warts 3–4 mm in diameter, primarily affecting young tubers. Over time, these warts dry out, the epidermis cracks, and a black powdery mass consisting of resting fungal spores is revealed inside.

Silver scurf. The causative agent is the fungus Spondilocladium atrovirens (class Deuteromycetes). The disease develops closer to spring, resulting in The formation of spots with a silvery sheen on the tuber surface. This condition is characteristic of potatoes grown in northern regions on peat soils. In the affected areas, the tissue becomes slightly depressed and acquires a metallic or silvery luster. Small black fungal sclerotia are visible On the surface of these spots.

Bumpy scab or oosporosis. The disease is caused by the fungus Oospora pustulans (class Deuteromycetes). Potato tubers are infected in the field, but symptoms are typically recognized only after 1–2 months of storage. Deep crater-like bumps appear on the tuber surface near the eyes. Under high humidity conditions in the storage facility, a grayish-white coating develops on the affected areas, representing fungal mycelium with small cylindrical conidia.

Rhizoctonia canker or black scurf. This disease affects potatoes, beets, carrots, rutabagas, cabbages, tomatoes, radishes, and many other crops. The causative agent in beets and carrots is the fungus Rhizoctonia violaceae (class Deuteromycetes). A dense, felt-like dark-purple fungal mat develops on the surface of the affected root vegetables. Gradually, this mycelial mat disappears, and the surface of the root vegetables becomes covered with numerous, very small black fungal sclerotia. The affected areas remain firm for a long time before eventually softening and rotting.

In potatoes, rhizoctonia is caused by the fungus Hypochnus solani (synonym Rhizoctonia solani). Affected tubers develop irregular black scabby bumps that are easily scraped off. During storage, the disease can progress, forming spots 1–3 cm in diameter with decayed tissue extending to a depth of 3–5 cm. The damaged tissue becomes fibrous, and dark sclerotia and mycelial growth form on its surface.

Cabbage rhizoctonia occurs in late cabbage varieties, particularly during rainy autumns. The cabbage leaves rot and detach from the stalk. Small black sclerotia form near the central leaf vein. In the late Stages of the disease, the leaves turn yellow, dry out, and take on a waxy appearance.

Black dry rot or alternariosis. The disease is caused by fungi of the genus Altemaria — A. radicina, A. brassica (class Deuteromycetes). It affects carrots, tomatoes, cabbages, and citrus fruits. A black spot appears on the crown of the root vegetable, after which the blackening spreads to the core, and the rot penetrates inward. The tissues of fruits and vegetables turn brown, then black and firm. Additionally, black ulcers form on the surface. In cross-section, the diseased black tissue of the root vegetable is sharply demarcated from healthy tissue, which is a key distinguishing feature of alternariosis compared to phoma rot, a disease where the diseased tissue is brown in color.

Phoma rot. Phoma rot affects HEAD cabbage, cauliflower, and most root vegetables: carrots, beets, turnips, rutabagas, radishes, etc. The causative agents are fungi of the genus Phoma (class Deuteromycetes). Specifically, Phoma tuberose affects potatoes, Phoma lindam affects cabbage, Phoma betae affects beets, and Phoma rostrupii affects carrots. Inside the roots during storage, the fungus forms dark brown or black pycnidia (groups of fruiting bodies). In cross-section, the affected root tissue is dark brown, dry, and partially hollow. When a pycnidium is crushed in a drop of water under a Microscope, a narrow ribbon of spores bound together by mucus can be observed.

White mold. This disease occurs in virtually all types of vegetable crops: cucumbers, cabbage, tomatoes, and root vegetables. The causative agent is the fungus Sclerotinia sclerotiorum. When developing on vegetables, it produces a dense white mold. After some time, the mycelium condenses into a film upon which black sclerotia up to 3 cm in size appear. During the formation of these sclerotia, transparent, glossy droplets of moisture are exuded from them. The flesh of the root vegetables softens and turns into a mushy consistency. The white mold pathogen spreads rapidly to other vegetables or root vegetables, potentially infecting an entire batch in a storage facility.

Grey mold. This rot affects various parts of fruits, vegetables, and berries, significantly reducing the yield of many cultivated plants. Grey mold is caused by the fungus Botrytis cinerea and related species. It causes particularly severe damage to strawberries during their ripening period. The berries become watery, develop brown spots, and are subsequently covered with a grey fluffy coating. Affected fruits turn brown, dry out, and mummify.

Botrytis cinerea frequently infects head cabbage, carrots, beets, as well as tomatoes, eggplants, peppers, and cucumbers. The surface of the cabbage heads becomes covered with a white fluffy mold, and the leaves turn slimy and decay. In beets, the infection begins at the tail end: the surface becomes covered with grey fluffy mold, and the tissue turns brown.

The fungus species Botrytis allii causes neck rot in onions. Dents appear near the neck of the bulb; the tissue softens and turns a dingy yellow. The surface of the bulb is covered with a grey mold coating, on which black sclerotia eventually form. Botrytis can reproduce across a wide temperature range: from —5 to 35 °C.

Clamp rot of beets is a disease caused by several different fungal species: Phoma betae, Botrytis cinerea, Trichothecium roseum, as well as representatives of the genera Penicillium, Aspergillus, Fusarium, and Rhizopus. Mold develops on the surface of the root crop, its color depending on the dominant species (white, grey, pink, olive-green, or black). The affected tissue also varies in consistency—dry, slimy, or macerated. The Development of the disease is promoted by elevated temperature and humidity in the vegetable storage facility.

Fruit rot or moniliosis. The disease primarily affects pome and stone fruits and is caused by the fungal species Monilia fructigena. The fungus features a well-developed multicellular mycelium with short conidiophores bearing chains of colorless, single-celled, lemon-shaped conidia. Small yellow-brown formations appear on the skin of the fruits, which fairly rapidly enlarge and turn brown. On the affected surface, the fungal conidiophores with conidia are visible as round grey pustules arranged in concentric circles. The fruit skin becomes hard and leathery, changing color from dark brown to blue-black. For the most part, the entire fruit is affected, eventually drying out or mummifying. In these mummified fruits, the fungus enters a resting stage—the sclerotium.

Soft rot. This develops during the storage of sweet cherries, strawberries, raspberries, and persimmons. The causative agent of the disease is the fungus Rhizopus stolonifer. Brown watery spots form on the affected fruits, which increase in size and become covered with a white felt-like coating bearing the black sporangia of the fungus. The active pectinases of R. stolonifer break down the transverse walls of cellular tissue, causing the supporting tissues to lose their Functions and Cell sap to leak out. The damaged tissues are easily further destroyed by other microorganisms, turning into a wet, mushy mass with a pungent odor.

Green mold or penicilliosis. This occurs most frequently in pome and citrus fruits. In apples, it is first detected as light brown Changes in the skin, after which white-grey mold ridges carrying green powdery conidia emerge from the softened pulp. The causative agent of green mold—Penicillium expansum—can infect only ripe fruits; therefore, unlike other rots, this disease occurs only after prolonged storage. A specific green mold on citrus fruits is caused by the fungi Penicillium italicum and P. digitatum. As a result of the mold's development, the fruit skin softens and presses inward. The pulp softens and acquires a bitter taste. At low storage temperatures (+1 and 0 °C), the development of P. italicum almost ceases. Penicilliosis is also widespread in vegetable crops, onions, and especially garlic.

Black rot of citrus fruits. This affects lemons, mandarins, and grapefruits during prolonged storage. The causative agent of the disease—the fungus Alternaria citri—appears at the Base of the fruit, from where it penetrates inside. The mycelium is initially grayish-brown and subsequently blackish-green. A black coloration appears inside the fruits, and the tissue softens.

Black rot of carrots is caused by the fungus Alternaria radicina, which forms dark grey depressed spots on the top and sides of the root crop, eventually turning into black ulcers. As the disease progresses, the root crop also blackens on the inside.

Olive mold rot or alternariosis of fruits is caused by the fungal species Alternaria tenuis. The pathogen penetrates the tissue through mechanical injuries. Olive-green mycelium proliferates on the fruit surface. The tissue beneath the mycelium turns dark brown or black, and the decayed areas become dry and hard.

Bacterial rots of vegetables are caused by certain species of phytopathogenic bacteria.

Wet bacterial rot of potatoes is caused by several bacterial species developing together: Pseudomonas syringae (synonym Pseudomonas xanthochlora), Erwinia carotovora, and others. Among them, Pseudomonas syringae is the most active. These are small, Gram-negative, motile rods that do not form spores or capsules. The disease manifests as the formation of spots beneath which the tissue turns yellowish; the boundary between the diseased and healthy tissue is blurred and exhibits a dark brown or even black coloration. At elevated temperatures, wet rot rapidly transforms potato tubers into a mushy, foul-smelling mass. At 15–20 °C, the affected tuber completely rots away within 5–6 days. To prevent the spread of the disease, it is recommended to maintain a storage temperature of about 1–2 °C and a relative air humidity of 80–85 %.

Brown rot of potatoes, or bacterial wilt, is a dangerous potato disease caused by bacteria of the species Ralstonia solanacearum (synonym Pseudomonas solanacearum). The pathogen enters the tubers from contaminated soil through injuries, as well as through Stomata or stolons. The symptoms of plant disease manifest as the wilting of leaves, which turn bronze, then curl and drop off. Cut sections of affected tubers show brown rings from which sticky slime oozes out. During storage, the affected tubers rot completely.

Bacterial rot of vegetables (carrots, beets, onions, turnips, cabbage, tomatoes, etc.) is caused by the bacteria Erwinia carotovora and Erwinia aroidae. The genus Erwinia belongs to the family Enterobacteriaceae and consists of Gram-negative rods measuring (0.6–0.9) × (1.5–5) µm, which are motile, do not form spores or capsules, and are anaerobes. The disease pathogen penetrates the root crop through wounds and cracks, around which dark spots soon form. Cracks frequently develop on diseased root crops, releasing a rotting mass teeming with bacteria. The disease can spread rapidly in storage, where root crops touching one another infect each other. The tissue of the root crop turns into a slimy mass emitting an unpleasant odor. The integumentary tissues persist longer than the internal ones and cover the decaying tissue for some time.

Sour bacterial rot of bulb scales. The causative agent of the disease is Burkholderia cepacia (synonym Pseudomonas cepacia). During growth in the field, one or two leaves of the plant change color from green to light brown. Then, a watery rot forms at the base and spreads to the tissues in the bulb neck region, making the green shoots easily pull out of the bulb. Several upper scales of the bulb may remain uninfected, which is a characteristic feature of this disease. The affected scales become slimy and translucent, changing color from pale yellow to light brown; they easily detach from the adjacent scales, causing the hard central scales to pop out readily when the bulb is squeezed. Affected bulbs often have a sour odor.

The main measures aimed at preserving the quality of fruits and vegetables and preventing microbial spoilage include:

✵ timely harvesting (unripe and overripe fruits and vegetables store poorly);

✵ storing only healthy produce;

✵ maintaining the required storage conditions (temperature and humidity);

✵ using a modified gaseous atmosphere composition;

✵ applying radurizing (irradiation with low doses of gamma rays) to preserve perishable fruits and vegetables.