Lots of science today! Phytophthora [fi-TOF-thir-a] is an organism [no longer considered a fungus] that has invaded world-wide. Many species exist. The one of concern in Davis and inland California attacks plants when ground conditions are HOT AND DAMP! (That is, high humidity and high temperature at the crown or roots for 48 hours.) Since we don't have rain here for eight months, the only way there can be enough moisture to allow Phytophthora to grow is if HUMANS apply water too often -- that is, they don't give the area time to dry out between waterings! Luckily, Don Shor has studied this organism extensively and helps us understand Phytophthora's life cycle and so learn how to reduce our risk. PS: That word "Phytophthora" literally translated to "plant death"!!! PPS: It's already here -- in the ground, on plant roots -- you can't avoid it if you garden.
Even old, established trees can be killed by Phytophthora if the watering changes. This photo is Dr Milton Hildebrand standing beside a cork oak he planted in 1949.
Don and Lois also talked about iron deficiencies and pH. The original question was about chlorosis -- yellowing of new leaves in citrus -- and what might cause that and how to fix it. The answer was wide-ranging. It turns out most mineral deficiencies in Davis are NOT caused by lack on minerals, but rather by conditions which keep the roots from taking them up. Mycorrhiza are organisms living in the soil and in roots which help plants extract minerals from the soil. These are GOOD things and exist in soils naturally -- with different species in different regions. So 15 years ago, when Lois said "If you move a wild blueberry plant into your garden, be sure to plant some of the wild SOIL with it," she was right! But only if it was in the same county. Shipping Michigan soil to California wouldn't help because the Mycorrhiza THERE are different than the Mycorrhiza HERE. Good news! -- the Mycorrhiza here work just fine with blueberry plants! And with other plants. No need to "import" or augment your native soil Mycorrhiza. (If you want to encourage them, just spread compost on TOP of the soil and they will do the rest.) The current version of the UCD Arboretim's newletter "The Leaflet" is at https://mailchi.mp/ucdavis/tree-issue
Don and Lois also got a wonderful email just saying how much a new listener enjoyed the show. Now THAT'S nice!!!