One focus of the working group on fruit diseases is the work on fruit tree phytoplasma diseases: apple proliferation (AP), European stone fruit yellows (ESFY) and pear decline (PD). In addition, the identification of Flavescence dorée-like phytoplasmas in the vicinity of vineyards (project: InvaProtect) is worked on.
The phytoplasmas are cell wall-less bacteria that only occur in the phloem of infected plants. They are transmitted by phloem-sucking insects (psyllids, cicadellids). We develop environmentally friendly control strategies against the further spread of these diseases. This starts with a good pathogen diagnosis and a molecular characterization of the phytoplasmas. Furthermore, we identify the transmitting insects and elucidate the exact transmission parameters. On the plant side, we try to use genetic and induced resistance to minimize the damage caused by the phytoplasmas.
Typical symptoms of apple proliferation are witches' brooms (Fig. 1) and enlarged stipules. The economic damage of the disease arises from the induction of small fruits, which makes it impossible to market the fruits. The phytoplasma (Candidatus Phytoplasma mali) is transmitted very efficiently by the leaf sucker Cacopsylla picta (Fig. 2).
Fig. 1 Witches' brooms
Abb. 2 Cacopsylla picta
The European stone fruit yellows ESFY is a quarantine disease that causes great economic damage by the death of the trees, especially in apricot and peach. Typical symptoms in summer are a chlorotic, conical leaf roll (Fig. 3) and in early winter a premature bud break. The phytoplasma (Candidatus Phytoplasma prunorum) is transmitted very efficiently by the leaf sucker Cacopsylla pruni (Fig. 4).
Fig. 3 Chlorotic leaf roll caused by ESFY
Fig. 4 Cacopsylla pruni
A closely related disease is pear decline (PD). It can lead to a more or less rapid death of the pear trees. However, symptoms such as reddening (Figure 5) are nonspecific and reliable detection requires a molecular test. The phytoplasma (Candidatus Phytoplasma pyri) is transmitted by various pear psyllids: Cacopsylla pyri (Fig. 6), Cacopsylla pyricola and possibly also by Cacopsylla pyrisuga.
Fig. 5 Reddening
Fig. 6 Cacopsylla pyri
Since phytoplasmas are not culturable, research must always be done on the entire infected plant or insect. To this end, plant tissue culture is established as a platform technology at AlPlanta. In this in vitro system, we test both the genetic resistance of apple root stock genotypes and the effect of bacterial and fungal endophytes (Figure 7).
Fig. 7 In vitro grafting of phytoplasma-infected plant on test plant
for the screening of genetic and induced resistance in the laboratory