Has half a cabbage turned brown? Or have the pea plants in your garden turned white?
If you look at the photo below, you will see that new (uninfected) growth is bright green, as it should be. Everything else on the plant looks bleached. That bleaching is caused by a fungal disease known as Fusarium wilt.
Similar to Verticillium wilt, Fusarium wilt is a common vascular disease in which a fungus (Fusarium oxysporum) clogs vascular vessels. It’s pretty much the coronary artery disease of the plant world. In addition to bleaching, common symptoms of Fusarium wilt include chlorosis, stunting, damping-off, brown veins, necrosis, and premature leaf drop.
This soil pathogen is found worldwide and it is spread by water splash, tools, and infected seeds and transplants. Fusarium wilt enters a healthy plant when germinating spores (mycelia) stab at the plant’s root tips and any damaged root tissue. That’s where the really amazing stuff starts to happen!
The Fusarium oxysporum fungus has no known sexual stage. Instead, it produces three different asexual spores: microconidia, macroconidia, and chlamydospores. Basically, the germinating spores (mycelia) inject themselves into a plant’s root system. From there, the mycelia move through the cells of the root cortex and into the xylem (a plant vein). Then, it starts producing the microconidia (asexual spores). The microconidia join the sap stream for a free ride to the rest of the plant. Eventually, there are so many microconidia that a vein is blocked. That’s when they germinate.
The vein blockage stops the plant form absorbing and moving nutrients, so the stomas close, the leaves wilt, everything looks bleached and it dies. As the plant dies, the fungus spreads throughout the plant and sporulates. [Cool word, right? It means “to produce spores”]
Plants affected by Fusarium wilt
Fusarium wilt attacks a variety of garden plants and the pathogens are specialized according to the victim. Fusarium wilt can attack peas, beans, and other legumes, tomatoes, tobacco, sweet potatoes, cucumber and other cucurbits, and even banana plants! When Fusarium attacks cabbages and other cruciferous vegetables, yellowing and browning leaves is the most common symptom. This particular form of is called Fusarium yellows.
Fusarium wilt controls
Once a plant is infected with Fusarium wilt, there is nothing to be done except remove the plant and toss it in the trash. Planting resistant varieties in the garden can help prevent Fusarium wilt, but crop rotation is not an effective control method. This is because the chlamydospores can hang out in the soil indefinitely. Some fungicides can be marginally effective.
Since Fusarium oxysporum prefers heavy, moist soil, aeration and adding compost to the garden can bring more oxygen into the soil. This reduces the welcome mat effect for many types of fungus. Ensuring proper drainage is the best way to avoid this garden menace.
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