Fusarium wilt of strawberry: symptoms and biology of the soil-borne pathogen

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The most common diseases in strawberries associated with damage to the root and vascular systems of plants of any age. Several pathogens are known, belonging to three genera: Rhizoctonia, Verticillium, Phytophthora.

Pathogens — Rhizoctonia fragariae Husain (sometimes other species as well).

Harmfulness. Manifests on plantations throughout the entire growing season, but the degree of its manifestation and symptoms may vary. Plant damage can reach 25% or more depending on climatic factors and agricultural practices; it usually has a focal character and is more often confined to areas with lower relief, which indicates the possibility of infection spread by groundwater and precipitation. Symptoms. Necrosis along the edges of the leaf blade, noticeably weak wilting of the affected leaf lobes. Leaves and petioles gradually turn brown, become dark brown, and die off. The leaf rosette collapses; diseased bushes sit (seem to press themselves to the ground). Wilting of plants usually begins from the phase of berry sizing and ripening, i.e., during the period of increased plant demand for nutrients and water. During this period, all plant leaves lose turgor and droop. Plant death occurs approximately 1.5 months after the appearance of the first symptoms of the disease. On cross and longitudinal sections of the roots of diseased plants, darkened (necrotic) areas of the vascular system are detected. The root collar and the main root rot.

575 => FR Fig. 195. FUSARIUM WILT OF STRAWBERRY PLANT r s y $37 < i. t. 14 \) e— a d i \ ya rn a —,: ei u G me 7 u - v > No y | `” 0h 9 4. 4” = shch RU ” 5 we 2” a ie.’, 2 5 d“ Biology of pathogens. The pathogens are soil-borne pathogens of broad specialization and develop in a very wide temperature range (from 5 to 35°C). They are also capable of infecting grain legumes, beet, tomatoes, potato, wheat, etc.

Fungal hyphae penetrate the plant through the roots and, in the process of their vital activity, produce toxic substances leading to necrosis of the vessels. In humid conditions, abundant white mycelium of the pathogen with dark burgundy clusters develops on the root collar of the affected strawberry plant. Microconidia are pear-shaped, spherical-ovoid, or lemon-shaped, unicellular, 4.4-6.8 x 5.7-7.6 microns in size. Macroconidia are sickle-elongated with three septa.

Source of infection — infested soil, planting material.

Protection measures. Most strawberry cultivars are not resistant to the disease. High-level crop management is necessary, increasing plant resistance to the pathogen. Plantations should be established on light soils using healthy planting material and organic fertilizers (50-100 t/ha) should be applied before planting. Strawberry crop rotations should include agricultural crops that are less susceptible to pathogens. In nursery plantations, all plants with signs of Fusarium wilt must be removed, and infected areas must be disinfected with a 4-5% formalin solution or 4-5% ferrous sulfate, followed by loosening.

Verticillium wilt. Pathogen — Verticillium albo-atrum Reinke et Berth, or V. dahliae Kleb. Symptoms. Strawberry of any age is affected. Symptoms of wilting develop from the periphery to the center of the bush. Old marginal leaves lose turgor, their petioles turn red, they spread out on the soil surface, turn brown, and dry out. Young central leaves become matte, chlorotic, and smaller. Affected bushes stop growing and then die completely. Root vessels and tissues at the base of the crowns die off; white and then gray mycelium with yellowish-brown microsclerotia is detected in them.

Biology of the pathogen. Conidial sporulation develops in humid conditions on affected parts of the plant in the form of whorled-branched conidiophores with oval smoky conidia. The pathogen survives in the soil as microsclerotia for up to 10 years and penetrates the plant through the roots. Mycelium, spreading through the vessels, causes their intoxication and mechanical blockage, which explains the symptom of wilting. Source of infection. The disease is transmitted to rosettes (runners) via stolons, and also through the soil. With severe soil infestation, runners selected from healthy plants can be infected (Beloshapkina, Govorov, 2000). In such cases, the disease is chronic, as not all infected rosettes show symptoms in the first year of planting. The fungus can persist in the plant for several years, and the disease is detected only under soil drought or violation of agricultural practices, i.e., under conditions of plant weakening and decreased resistance to the wilt pathogen. Plant resistance to the pathogen. The following strawberry cultivars are severely affected by Verticillium wilt: Festivalnaya, Zarya, Krasavitsa Zagorya. Relatively resistant cultivars are Feya, Senga-Sengana, Estafeta, Zenit, Sudarushka. Protection measures. 1. Establishing a strawberry plantation is carried out using healthy planting material. It is recommended to grow only the strawberry cultivars most resistant to wilt. 2. Strawberries should not be placed after cucumbers, other cucurbit crops, or tomato. 3. Diseased plants must be removed from the site along with rosettes and destroyed. If necessary, after soil analysis for Verticillium using the willow stick method, it is recommended to disinfect the soil with Basamid granules. The preparation is applied in the autumn before spring planting. It is tilled into the soil to a depth of 15-20 cm, and then the soil is watered abundantly. Phytophthora root rot of strawberries

Pathogen — Phytophthora fragariae Hickman, var. fragariae Wilcox Duncan. Harmfulness. A quarantine species for Europe. Phytophthora root rot of strawberry is one of the most dangerous diseases of these crops. Affecting individual plants in the year of planting, under certain circumstances, the disease can acquire the character of an epiphytotic by the end of the second year. Phytophthora develops particularly actively on clayey, poorly aerated and poorly drained soil, at low temperatures (10-17°).

Prevalence. It is quite often noted on strawberry and wild strawberry in many European countries, is widespread in the USA, and is registered in a number of other countries. There are a number of publications on the presence of the disease in the territory of CIS countries and in Russia.

Symptoms. On the aerial parts of the strawberry (wild strawberry), there is a delay in development, the appearance of smaller leaves with a grayish tint, stunted growth of bushes, premature aging of leaves, and their yellowing or reddening. Plants may die even before fruiting or form a small number of small fruits. In some cases, browning and death of runners, and drying and blackening of berries are observed.

On the roots, the rot manifests itself most clearly in late autumn: when digging up affected plants, a poorly developed root system is visible due to the rotting of lateral feeder roots. In areas of decay, the roots have a grayish or brownish color, which gives the characteristic symptoms of "rat tails." Upon a longitudinal section of the upper (non-rotted) parts of these roots, stele cylinders are visible. Their color varies from wine-red to brick-red.

Biology of the pathogen. A narrowly specialized parasite, it affects only cultivated strawberries. The pathogen has mycelium, zoospores, and oospores in its development cycle. The latter are formed in the central cylinder of the root throughout the summer, germinate after the destruction of the roots, and are able to persist in the soil for up to eight years. The mycelium develops in the root wood and usually grows slowly. Optimal temperature for development is 18-22°; at 30°, growth ceases. Zoosporangia on zoosporangiophores are formed on the surface of affected roots in damp soil or in water. Zoospores infect the young lateral rootlets of the mother plant or runners.

The source of Phytophthora infection in strawberries is oospores in the soil and infected transplants. Strawberries are more severely affected at high soil moisture, as well as with the application of high doses of nitrogen fertilizer. Potash and phosphorus fertilizer reduce the development of the disease. Mulching of aisles with sawdust promotes the development of Phytophthora due to an increase in soil moisture under the mulch. Fig. 196. STRAWBERRY PHYTOPHTHORA

Protection methods. Timely diagnosis and implementation of protective measures. Examination of strawberries is carried out in early summer, after the flowering of the crops, when a complex of symptoms appears on the aerial organs of the plants. When inspecting plants, one should pay attention to stunted plants with signs of wilting. Upon detection of signs of Phytophthora, it is necessary to dig up the affected plants (at least 10 specimens) with the root system and the soil sticking to it, pack them in several layers of paper, label them, and send them to a quarantine laboratory for analysis.

Disinfection of the soil substrate and the use of healthy planting material are the basis for preventing this dangerous disease. The use of plant protection products has not been developed in detail. There is experience in using a mixture of metalaxyl with copper oxychloride or mancozeb. The preparations are applied to the rows, closer to the base of the plants, in autumn or spring. 2.16.2.4. Phytophthora Pathogen — Phytophthora cactorum (Leb. et Cohn) Schroet.

Host plants. A widely specialized pathogen, it affects over 120 species of plants, which include fruit and vegetable crops, potatoes, floral and some ornamental crops, and many weeds.

Harmfulness. The fungus affects the root system and the aerial part of the plant, causing a wide variety of symptoms. The loss of strawberry berry harvest due to this disease reaches 20% or more.

Symptoms. During the growing season, they can be different on different parts of the plant. Affected bushes are characterized by some spreading of leaves from the center to the periphery. The bush suddenly wilts rapidly. First, the lower leaves wilt, turning upside down and lying on the soil. This occurs as a result of the rotting of leaf stalks, flower stalks, and the root neck, which is preceded by the appearance of brown encircling spots at the base of the strawberry bush. At the same time, the root system of the bush dies off. Necrosis of individual roots in a well-developed bush does not cause its death, but the leaves develop stunted, deformed, on thin, long stalks.

Under conditions of high air humidity, diffuse, irregularly shaped brown oily spots appear on the leaves, which dry out in dry weather. Older leaves become stiff, their edges curl downwards, and necroses appear on the veins, bending the leaf blade (or its lobe) at an angle. The rhizomes become covered with deep ulcers and, when subjected to mechanical stress, easily disintegrate and detach from the roots. Young rosettes form on short runners. Their lower leaves are small and deformed, while the upper ones are stiff and corrugated. Brown necrotic spots appear on the lower part of the rosette stem. In terms of symptoms, late blight begins to resemble nematode infestation.

The disease affects strawberries at all stages of development, starting from the flowering period. The first sign is necrosis of the pistils — the center of the flower darkens, and the affected fruit set becomes covered with brown spots, stops growing, and dries out. On green berries, brown spots gradually cover the entire surface, causing it to become leathery and firm.

On ripening fruits, spots form on the sides: they are light, slightly depressed, sometimes with a violet tint. The consistency of the pulp becomes firm and rubbery, and a cross-section reveals a darkening of the core, extending from the peduncle. The affected berry acquires an unpleasant taste and smell.

The pathogen persists in the form of spherical light-brown oospores in the soil, plant residues, and in all infected plant organs. In spring, the overwintered oospores produce zoospores, which cause the primary infection. Under high humidity, a faint bloom appears on the berries and other diseased parts of the bush — this is a sign of asexual sporulation of the pathogen.

• Temperature for infection — 18-22°
• Relative air humidity — 95-100%
• Average diameter of oospores — 29 μm
• Concentration of Aliette working solution — 0.2%
• Number of sprayings per season — 3

Prevention and chemical plant protection

Plantation protection is based on strict control of the phytosanitary status. When planning protective measures, it is necessary to consider previous phytosanitary activities, quality of transplants, and the proper crop rotation in the strawberry rotation system. Since the late blight pathogen can persist on weeds, regular control of weed vegetation is an essential maintenance element.

For chemical protection of plantings, the preparation Aliette is used in the form of a 0.2% working solution. Spraying is carried out strictly according to the plant development phases to achieve maximum effectiveness. The protection scheme includes three sequential treatments:

1. The first spraying of the plants is carried out during the budding period.
2. The second spraying is performed immediately after flowering.
3. The third spraying is carried out at the end of the growing season.