Disease Control-74

 

 

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Project Leader and Principal UC Investigators

R.K. Webster

C.M. Wick

R. Keim

J. Bolstad

S. Ferreira

R. Jones

 

Seed Rot and Seedling Disease: Rice seedling diseases caused by Achlya Klebsiana and Pythium species play an important role in the establishment of uniform optimum stands of water-sown rice in California. These diseases, although prevalent throughout the rice producing areas of California, are generally more severe when temperatures during the planting season are cool and unfavorable for the growth and establishment of seedlings.

þ Seed treatment with fungicides has been shown to be an effective control for rice seedling disease. Seed processors throughout the state have become equipped to provide this service for California growers. The benefits from fungicide seed treatment in water-sown rice are most obvious during the early planting season when environmental conditions are often unfavorable fur germination and seedling growth. Primary benefits include: (1) cost savings from reduced seeding rates and planting far exceeding the cost of treatment per acre, (2) more uniform stands and subsequent higher yields, and (3) elimi
nation of the occasional need to replant some fields. In the event Difolatan® is removed from the approved list as a rice seed treatment fungicide, effective replacement chemicals have been identified.

Stem Rot: Stem rot of rice, caused by Sclerotium oryzae, is endemic over a large portion of the California rice-producing area. The causal organism overwinters in the form of sclerotia either in the soil or in association with crop residue. At planting time, sclerotia float to the water surface when fields are flooded and infect plants as they emerge through the water.

Initial infections appear as small dark lesions on the leaf sheaths at the water level. These enlarge and penetrate the stem as the season progresses. Infections that occur early in the season often kill tillers or result in smaller panicles with unfilled grains of poorer quality. Infections occurring later usually do not kill the tiller but result in smaller panicles and enhance lodging, resulting in additional yield losses due to decreased harvester efficiency. The average loss due to stem rot in California is estimated to be between 5-8% annually, with much higher losses occurring in certain areas and individual fields where the disease is more severe. Here in capsule form are the research findings about this very costly disease:

  1. An understanding of rice soil microflora and their possible exploitation in minimizing stem rot inoculum has been obtained.
  2. It is now apparent that differences in resistance to stem rot between genetic lines is heritable and ultimately can be transferred to new varieties in the plant breeding program. Reliable methods for quickly screening large numbers of progeny have been developed. Superior sources of stem rot resistance have not yet been found.
  3. Burning of rice residue presently is our best means of minimizing stem rot disease, and growers with serious infections should be permitted to continue this practice. Even if engineering research should be successful in developing an economical system to remove most of the straw from rice fields for useful purposes, it is probable rice research still will have to develop economical solutions to the stem rot problem. Perhaps the greatest benefits to be derived from results obtained through this research are that it is now possible for the rice industry to respond from a factual basis to queries and critics regarding burning of rice straw.
  4. þThe probability exists that stem rot could be controlled by development of more resistant varieties or by fungicide applications. The use of these methods will be pursued with the expectation that best results will be obtained when either or both are used in conjunction with burning residue.

 

This UCD greenhouse experiment conducted by plant pathologist Dr. R. K. Webster dramatically shows the devastating effects of stem rot on rice. Each basin plot is 18" x 18". That on the right and those in the background were not inoculated. The one on the left was inoculated with the disease.

 

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