In This Issue

Bacterial Canker and Phytophthora Root Rot

There are two primary culprits that cause spring time tree death: Bacterial Canker and Phytophthora Root Rot (also called crown rot). Bacterial canker usually appears on young trees and is associated with the presence of ring nematode. Trees growing in sandy soils are most often affected, although the disease can occur in heavier soils. Symptoms include shoot and scaffold death and gumming. It is common for just one limb or for only part of the tree to be affected. Affected wood will almost always have a sweet-sour smell to it. The roots of the tree are nearly always unaffected initially, and it is common for numerous rootstock suckers to push new growth, even though the scaffolds above them are dead or dying. Control methods include: removal of all old roots in the area, adjusting soil pH to the proper range, preplant fumigation, maintaining good tree vigor through proper irrigation and fertilization, postplant nematicide treatment, and delaying pruning until after bloom.

Phytophthora root rot can affect trees of any age, but also is common on young trees. The phytophthora fungus invades the root system of the tree and can eventually cause tree death. Orchards which receive too much water in the spring or in the fall - either from overirrigation or rain - are most susceptible to this disease. Symptoms include: leaf yellowing, overall scaffold weakening and decline, followed by eventual tree death. The best diagnosis is to cut into the tree trunk just below the soil surface. If phytophthora is present, the rootstock will be brown and decayed, and quite often slippery. It is common for vascular system of the aboveground shoots to be unaffected. Control methods include: planting trees on a berm and keeping the graft union above ground, use of proper rootstock for the soil type, and most importantly, proper irrigation management. There are registered chemical products which will help control phytophthora if the disease is detected before becoming too severe.

In summary, the symptoms of these two diseases are almost exact opposites of one another. The following table should help in diagnosing between the two.

Irrigating Young Trees

Irrigating young trees in the first few years of orchard establishment is a critical practice. The goal is to maximize tree growth and root expansion without stressing the trees or waterlogging the root system. Since the roots are constantly growing, it is difficult to know just where and how much water should be applied.

Using the weighing lysimeter at the Kearney Agricultural Center, we have obtained young tree water use values over a three year period. These data were used to develop the numbers shown in Table 1, page 3. To apply this information to a given orchard, make a rough measurement of the 3 dimensions (height, E-W width, N-S width) of an average tree in the field. Multiply these together to give an estimate of tree volume. The table gives estimates of the amount of water used during the months of the season by trees of varying volumes. Units are in gallons per tree per week. For instance, a tree with a volume of 200 cubic feet will need 67 gallons of water each week in July. Values within the table may need to be altered for any given orchard because of the following factors.

• Irrigation efficiency. The table assumes high efficiency since our trees were irrigated with multiple drip emitters per tree. If microsprinklers are used, there could be more soil evaporation and water application which goes beyond the root zone; such trees could possibly require 10-20% more water. For most flood or furrow irrigated orchards, application efficiency is usually poor and more water may be required.
  
• Current weather conditions. Since the table is based on long term temperature averages, abnormally hot or cold spells should be taken into account when scheduling irrigations.
  
• Soil type. On very sandy soils where water may be leaching beyond the root zone, extra water will likely need to be applied.
  
• Cover crops & weed growth. The values in the table were derived from trees with no weed growth. Any other plant growth in the orchard will significantly increase the water requirements.

Leaf Sampling

June and July are the ideal time to collect leaf samples. While I do not believe that this needs to be conducted every year, leaf sampling should be performed on a 2 to 4 year basis. The figures generated from these samples can then be used to customize fertility needs to particular orchards. The following tables should be of assistance in determining potential nutrient problems and in interpreting leaf analysis results. Please contact the office if you have questions regarding leaf sampling.

¹ Percent N in August samples can be 0.2 - 0.3% lower than July samples and still be about equivalent. Nitrogen levels higher than maximum can adversely affect fruit quality.

² Potassium levels between deficient and adequate are considered "low" and may cause reduced fruit sizes some years. Potassium fertilization is recommended for "deficient" orchards but only test applications for "low" K orchards.

³ Excess Na or Cl causes reduced growth at levels shown. Leaf burn may or may not occur. Confirm salinity problems with soil or root samples.

Variety Display and Research Update Meeting Schedule

• Tuesday, June 15, 1999
  8:00 AM to 10:00 AM
  
  Featuring:
  
  • Display of New Stone Fruit Varieties, Local Nurseries and Plant Breeders
  • Dwarfing Rootstocks for Stone Fruit, Ted DeJong, UC Pomologist
  • Pest and Disease Update
  
  
  
• Tuesday, July 13, 1999
  8:00 AM to 10:00 AM
  
  Featuring:
  • Display of New Stone Fruit Varieties, Local Nurseries and Plant Breeders
  • Postharvest Practices for Improving Fruit Quality, Carlos Crisosto, UC Postharvest Physiologist
  • Pest and Disease Update
  
  
  
• Tuesday, August 17, 1999
  8:00 AM to 10:00 AM
  
  Featuring:
  • Display of Varieties
    
  • Topics in Tree Nutrition
    
  • Pest and Disease Update

These meetings are free and all interested people are encouraged to attend. For further information contact: Kevin R. Day (559) 733-6485