has answers regarding the effects and nature of juglone. It does, however, state that the statements made are specifically intended for a state other than Texas. I also found:
states the following:
Plants produce chemicals which can inhibit the growth of other plants. These chemicals are dissolved out of the leaves when it rains and are washed down to the ground below, reducing the growth of plants under the tree[18, 20, 159]. The roots also produce substances that are toxic to many plant species, especially apples (Malus species), members of the Ericaceae, Potentilla spp and the white pines (certain Pinus spp.). Beans, peas and tomatoes are also particularly sensitive to these secretions and will not grow in the rooting zone. Trees cast quite a dense shade so, along with their other anti-social tendencies, are not very friendly trees for a woodland garden[K].