14 MISC. PUBLICATION 541, U. S. DEPT. OF AGRICULTURE 



At this stage no mycelium or spores are visible. Decay lesions 

 enlarge rapidly, soon involving large portions of the stalk. Grayish, 

 smoke-colored mycelium and grayish-brown, granular masses of 

 spores on the larger lesions are characteristic signs of this disease 

 (pi. 8, B). Most infections take place at the bases of the leafstalks 

 where wounds are made in harvesting, but they may also occur in 

 apparently normal tissue. 



Kefrigeration temperatures of 40° to 45° F. will retard but not 

 stop the development of gray mold rot during transit. 



(See also Globe Artichokes, Gray Mold Eot, p. 6.) 



Phytophthora Rots (Foot and Crown Rots) 



(Phytophthora spp.) 



OCCURRENCE, SYMPTOMS, AND EFFECTS 



In all regions where rhubarb is grown commercially apparently one 

 or more species of Phytophthora may cause serious diseases. Although 

 different organisms are involved as causal agents the disease symptoms 

 produced are very similar and for practical purposes may be con- 

 sidered as characterizing one disease. 



The phytophthora diseases, often referred to as foot and crown rots, 

 are primarily field troubles, but occasionally infected stalks reach the 

 markets. Bacterial soft rot follows these rots so closely that by the 

 time diseased stalks reach the market the former appears to be the 

 more important decay. For this reason these two diseases often have 

 been confused on the market. 



The first indication of phytophthora rots usually is the wilting of a 

 few leaves. A watery, greenish-brown, sunken lesion at the base of 

 the leafstalk indicates the point of invasion by the causal fungus. 

 A rapid decay follows infection, and the stalks may collapse within 

 24 hours. The pathogen progresses from the diseased stalks into the 

 roots, causing a brown decay, and from there spreads upward into 

 other stalks and buds. Secondary bacterial infections usually cause 

 immediate decomposition of affected plants. 



CAUSAL FACTORS 



The pathogens responsible for most of the foot and crown rots of 

 rhubarb are Phytophthora parasitica Dast. and P. cactorum (Leb. and 

 Cohn) Schroet. However, a closely related fungus, Pythium ultirmim, 

 and other species of Pythiwm have also been found to cause serious 

 root, crown, and stalk rots in some localities. All these fungi produce 

 swimming spores (zoospores) , and for this reason the severity of in- 

 fection is directly correlated with the amount of rainfall. Warm, wet 

 weather is especially favorable for the development of the foot rot 

 caused by Phytophthora parasitica, since it grows best at about 86° 

 F. The minimum temperature for growth of this fungus is 55° and 

 the maximum 97°. P. cactorum, which commonly causes crown rot, is 

 more likely to be found farther north or to occur during cool, wet 

 seasons. Its minimum temperature for growth is 43°, the optimum 

 77°, and the maximum 92°. 



Infections by any of these organisms may take place in uninjured as 

 well as injured stalks. 



