The Control of the Fusarium Wilt of Orchids

Symptoms

• This disease causes severe yellowing of the leaves, which initially stay erect. As the fungus attacks the orchid's vascular tissue, they begin to collapse at the leaf stalk, then fall in order from oldest to youngest. Over time, the entire plant will wilt. If cut, plant stems show brown and black lines running through them in all directions and produce an unpleasant rotting smell. Plants infected in the seedling stage may grow slowly, with yellowish new leaves. The rhizome in infected orchids develops a pink to purple discoloration which eventually spreads through the whole rhizome and into the pseudobulbs. Seriously infected plants can die within three to nine weeks, according to the Houston Orchid Society, while plants with minor infections may take up to a year to die.

Causes

• Fusarium wilt is caused by two major fungi: Fusarium oxysporum and to a lesser extent, Fusarium solani. According to the Staug Orchid Society and HOS, growers often accidentally spread these pathogens by pruning their orchids with non-sterile cutting tools. Symptoms often develop in plants suffering from heat stress or in high humidity conditions. Heavy fertilizer applications can also encourage orchids to develop symptoms of fusarium wilt.

Prevention

• To prevent fusarium wilt in orchids, sterilize all cutting tools after each use, avoid reusing soil, and sterilize all pots before reuse. Keep plants healthy and reduce stress levels as much as possible. Closely monitor young plants transplanted in the heat of the summer, especially those transplanted from very small pots or community pots. Water these plants regularly to reduce water stress and keep fertilizer application to a minimum.

Treatment

• Treat fusarium wilt in orchids as soon as symptoms appear. Apply chlorathalonil fungicides to protect the plant from developing this disease and azoxystrobin fungicides to suppress fungal activity in already-infected plants. If a purple band appears in the rhizome or pseudobulbs, cut away the infected parts and repot only the part of the plant that shows no discoloration. Sterilize cutting tools between cuts to avoid transmitting the disease to undamaged parts of the plant.