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Desert Gardener: Fighting disease of young vegetable plants

September 10, 2001|By Tom Turini, University of California-Imperial County Cooperative Extension

Winter vegetables such as broccoli, carrots, cauliflower, cabbage and leaf lettuce can be sown between now and December. These vegetables are perfect for backyard gardens. However, there are several fungi that can kill seed and weaken or kill young seedlings.

Several fungi cause seedling disease and more than one may be infecting one plant. The fung, Rhizoctonia solani and Pythium spp. commonly cause seedling disease of many plant species. Other fungi, such as Macrophomina phaseolina and Fusarium spp. can also attack young plants. These fungi are common in our soils and under the right conditions can cause substantial damage to your young vegetable plants.

Seedling diseases may occur at different stages of a plant's development. A pre-emergent damping off occurs when the seed rots before germination or the seedling dies before it breaches the soil surface. Post-emergence damping off occurs when the seedling breaks through the soil surface and then succumbs to a fungal pathogen. When older seedlings are infected, the plants are weakened but rarely killed.


Regardless of what fungus does the damage or when the fungus kills the plant, the result of a fungal attack before or soon after emergence is a thin, uneven plant stand. It is important to note that seedling disease is not the only possible cause of poor stand. When seedling diseases are involved, underground plant parts are discolored. The roots may have a water-soaked appearance or the smaller roots may be rotted off.

When fungi attack older seedlings they can kill the outer layer of cells around the stem. This damage appears as a darkened constricted area on the stem near the soil line. As the seedlings continue to develop, fungi may continue to kill root tips and girdle stems, but the chances that the plant will die decreases quickly after emergence. When the fungi attack older plants the result is stunted rather than dead plants.

Control: Proper fertilization and irrigation will reduce the chances of these diseases becoming a problem. A plant can outgrow the disease when conditions favor rapid germination and plant growth.

Prepare soils so that water does not remain on the soil surface for long after irrigating. Insure there are no compaction layers and the area is free of low spots where water will collect. In addition, by planting seeds on raised beds, you are reducing the chances the seedlings will be exposed to waterlogged conditions.

Avoid planting into soil that contains plant materials that have not decomposed. Some potential pathogens thrive on organic material. In addition, as the plant material breaks down, it will emit chemicals that

can directly damage plants.

Don't plant too deep. Seedlings are most susceptible to damping off between seed germination and emergence. Deeply planted seed will be at this susceptible stage of development for a longer time than if the seed were planted at the correct depth.

For carrots, seed should be placed in a shallow groove and left uncovered. Lettuce seed should be placed in a grove and covered with a thin layer of soil. Broccoli, cabbage and cauliflower seed and seed of similar crops should be planted one-eighth to one-fourth inch deep.

When the soil temperatures drop, seed germination is slowed as is seedling growth, and plants are even more susceptible to seedling diseases. However, there are several fungi common in desert soils that can cause damage at high soil temperatures, particularly in wet soils.

Many seed companies will treat seed with fungicides. These chemicals can help to protect seedlings from early disease problem, which will reduce the chances of seedling death and allow plants to grow more vigorously at early stages of development.

However, not all fungicides protect against all fungi and seed treatments do not provide 100 percent protection under all conditions, so it is important to take other precautions even if the seed is treated.

The amount of damage caused by seedling disease depends upon susceptibility of the plant, the degree of infestation of the soil by the pathogen(s), environmental factors and the effectiveness of control measures.

In general, seedling diseases are favored by excessively wet soils, compacted or poorly drained soils and temperatures or conditions that do not favor germination and growth of the plant.

By properly preparing the soil, using treated seed and planting at the appropriate depth, the likelihood that your garden will produce plenty of good quality vegetables is maximized.

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