Despite by Fusarium oxysporum f. sp. lycopersici (Sacc.)

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Despite development of information and
achieving remarkable improvement in the management of plant diseases, plant
pathogens still cause many losses of the products. Soil-borne pathogens are
affected by physical, chemical and biological properties of the soil. The
productivity of vegetable crops is very low due to several biotic and abiotic
factors. Among biotic factors diseases caused by fungi, nematodes and viruses
are the major ones (Wodirad et al.,
2009). Root-knot nematode,
Meloidogyne javanica is considered as one of the major plant pathogens
infecting some strategic plant products, such as tomato (Hillocks, 2002).
However, many constraints affect
productivity and quality of tomato among which diseases play a salient role
(Pritesh et al., 2011). The wilt diseases are caused by bacteria (Pseudomonas
spp.) and fungi (Verticillium and Fusarium spp.) (Mardi et
al., 2002). Tomato Fusarium wilt is considered as one of the most
important diseases of tomato both in field and green house-grown tomatoes
worldwide (Sheu et al., 2006; Amini et al., 2010; Abdel-Monaim,
2012).

2.4.
Fusarium wilt of Tomato (Fusarium
oxysporum f.sp. lycopersici)2.4.1.The Causal  pathogen

Fusarium wilt of tomato is caused by Fusarium oxysporum f. sp. lycopersici
(Sacc.) which is a soil borne plant pathogen in the class Hyphomycetes
causes Fusarium wilt specifically on tomato. The fungus consists of serious
pathogenic strains that infects plants roots at all stages of plant growth
causing economic losses (Houssien et al., 2010). The pathogen penetrates through the roots mainly
through wounds and proceeds into the vascular system leading to the collapse of
the system. This destroys the root of the plants causing reduction in yields of
up to 40% especially on susceptible varieties when the soil and air
temperatures are high (Suarez-Estrella et al., 2007). Apart from causing diseases, they
colonize outer cells of roots as harmless endophytes after the pathogen has
killed the root tissues and others live as saprophytes in soil (Burgess et
al., 2008). The Fusarium fungus is a known pathogen of tomato plant
which is present in all important tomato growing regions of the world and
produces three types of asexual spores; microconidia, macroconidia and
chlamydospores (Mohammed, 1990). Some strains of Fusarium oxysporum are
not pathogenic and may even antagonize the growth of pathogenic strains and can
be used as biological agents (Fravel et al., 2003)

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2.4.2.
Biology, life cycle and symptoms of Fusarium wilt pathogen

Based on the structure on which
conindiogenous hyphae are borne they are classified under the sub class
Deuteromycetes. Fusarium oxysporum has no sexual stage (Booth, 1971) and
it produces three types of asexual spores: microconidia, macroconidia and
chlamydospores (Nelson et al., 1983). Conidia are produced on the
monophialides and in the sporodochia, and they are generally scattered loosely
over the surface of the mycelium (Griffin, 1994). Microconidia are the most abundantly
produced spores, they are oval, kidney shaped and they are produced on aerial
mycelia. They are also uninucleate and they germinate poorly and variably. Macroconidia
have three to five cells that have curved edges.

The pathogen infects healthy plants
as mycelia or by germinating spores that penetrate plant root tips, wounds or
lateral roots. The mycelium then advances intracellular through the root cortex
into the xylem. In the xylem the mycelium produces microconidia. The
microconidia then enter the vascular system and they are transported upwards
through the sap stream. At the point where the flow of the sap stops, the microconidia
germinate. The spores and the mycelia block the vascular vessels preventing the
flow of vascular fluids, reducing water and nutrient up-take. Generally, the
fungus causes vascular wilt by infecting and growing internally through the
cortex to the stele (Bowers and Locke, 2000). The infected plant transpires to
a greater extent than the water it is able to transport and the stomata close,
the leaves wilt and eventually the plant dies. After the death of the plant the
fungus invades all the tissues, sporulates and infects neighbouring plants
(Kutama et al., 2016).

Symptoms
of attack first appear as slight vein clearing on the outer portion of the
young leaves followed by epinasty of the older leaves. This symptom often
occurs on one side of the plant or on one shoot. Successive leaves yellow wilt
and die, often before the plant reaches maturity. As the disease progresses,
growth is typically stunted, and little or no fruit develops. If the main stem
is cut, dark brown streaks may be seen running lengthwise through the stem (Bawa,
2016). The browning of the vascular system is characteristic of the disease and
generally can be used for its identification (Mui-Yun, 2003).On the outside of
affected stems, white, pink or orange fungal growth can be seen especially in
wet conditions (Ajigbola and Babalola, 2013).

Categories: Management

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