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Disease Cycle and Epidemiology

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Disease Cycle/Epidemiology

Carpogenic germination of sclerotia
Following a period of conditioning in a near-saturated, relatively cool environment (10 to 20°C), soilborne sclerotia of S. sclerotiorum located within 2 cm of the soil surface produce apothecia and ascospores. Most frequently, this occurs after a significant rain or irrigation event, and is aided by a shaded, slow-drying soil surface. Ascospores are forcibly discharged (Fig. 14) and moved by air currents up to susceptible tissue. Prolonged periods of leaf wetness (16 to 48 hours) with temperatures in the range of 12 to 24°C promote ascospore germination and infection. Because the ascospores of S. sclerotiorum lack the energy necessary to infect healthy host tissues directly, a highly susceptible ‘weak’ host tissue, such as flower petals or senescing leaves, is typically the first tissue to become infected. Colonized ‘weak’ tissues then serve as a food base to grow mycelium with sufficient energy to penetrate healthy plant parts. As a consequence, major infection events often coincide with bloom and post-bloom periods, as senescing petals commonly drop and lodge in branch axils or adhere to leaves, petioles, stems and developing fruits. With adequate wetness, mycelium moves from the colonized tissue into the vigorous host tissues of stems, leaves, pods, etc. Later in the disease process, sclerotia form, either on the plant surface or within stems and other plant parts. As the plant or plant part dies, the sclerotia fall to the soil where they can survive for multiple years.

In some cases such as cabbage plants and tobacco seedlings, ascospores of S. sclerotiorum are able to infect plants through wounds. In these cases, mechanical or chemical injury, insect feeding or freezing can increase the incidence and severity of white mold.

Although S. trifoliorum also germinates carpogenically, the disease cycle of alfalfa crown and stem rot is unique. Sclerotia of S. trifoliorum oversummer in soil. In the fall, dropping soil temperatures (10 to 15°C) and increased soil moisture cause the sclerotia to germinate to produce apothecia and ascospores. Ascospores are blown to nearby plants, and under cool wet conditions can infect the leaves directly, forming leaf spots that expand over the winter. A soft rot develops, and if the crop was fall-planted, the seedlings can be killed. When established plants are affected, often only stems wilt and die, while the crowns survive. Mycelium spreads over the plants and down to the soil surface. When host tissue is depleted, or when weather turns warm and dry, sclerotia form on the diseased tissues.

Myceliogenic germination of sclerotia
Hyphal germination of sclerotia occurs when soil is wet and cool; optimal conditions for germination are near-saturated wetness with soil temperatures of 12 to 24°C. These conditions are created by extended rainy periods, or by irrigation events in combination with soil shading due to closure of the crop canopy. Plant infection following this type of germination is favored by similar soil conditions, and is limited to roots, crowns and other plant parts located within 1 to 2 cm of the sclerotia.

Disease epidemiology
White mold is a monocyclic disease, meaning that it has only a primary cycle of infection, and once the host tissue is diseased, it is not contagious (i.e., spore-producing) within the same season. Plant to plant spread of S. sclerotiorum and S. minor may occur occasionally, but this usually is a rare occurrence.

Most severe disease situations in a field develop from sclerotia that are local within the field soil. In the case of carpogenic germination of sclerotia, ascospore releases into the air typically occur at ground level under closed canopies, and consequently, sharp foci of infection are often observed within the field. Nonetheless, infections initiated by wind-disseminated ascospores from another location have been observed occasionally.

Introduction of the pathogen from outside sources also may occur by contaminated machinery, mass movement of water, and by wind-blown crop debris. In some hosts, seed may be internally infected with mycelium of Sclerotinia, or inadequately cleaned seed may be contaminated with seed-sized sclerotia. Numerous broadleaf weeds also are hosts of white mold, and may contribute to a persistence of sclerotia within a field.

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