August 31, 1912 



HORTICULTURE 



331 



PRESENT KNOWLEDGE OF SWEET 



PEA DISEASES AND THEIR 



CONTROL. 



A paijer by J, .T. T;iiilii'i]liaiis, lielawai-e 

 College Ajji'ienltural l^xporiuieiit Sta- 

 tion, Newark, Del., read before the 

 American Sweet Pea Society, 

 Boston, July 1.3, 1912. 



{Continued from page Sj) 



Diseases of the Sweet Pea. 



In dealing with the diseases of the 

 sweet iiea, we have to recognize two 

 classes: 1. Disease of the sweet pea 

 grown under glass. 2. Diseases of the 

 sweet pea grown outdoors. 



Under glass, sweet peas are not as 

 extensively grown as outdoors. The 

 time is undoubtedly rapidly ap- 

 proaching when the market will call 

 for more sweet peas in the winter. We 

 have no information as to how many 

 are actually engaged in growing winter 

 sweet peas, nor of the successes or 

 failures these growers meet. We know 

 of a few growers who have attempted 

 to grow sweet peas under glass but 

 who have failed. For the benefit of 

 the rest of us, we will here consider 

 some specific cases in order to bring 

 to your attention some of the possible 

 diseases which may trouble sweet peas 

 under glass. A grower (I cannot give 

 you the name) erected a forcing house 

 for the purpose of growing winter 

 sweet peas. His first attempt was a 

 failure. His plants began to die in 

 spots and only here and there did a 

 plant remain alive. The grower soon 

 complained to his seedsman and the 

 latter kindly referred him to us. We 

 immediately asked for a comjilete his- 

 tory of the trouble and for specimens 

 of diseased plants and soil. The fol- 

 lowing is the history of the trouble as 

 told by the grower himself: "The 

 house the peas are in was built last 

 summer, on a piece of ground which 

 had been in grass for over five years. 

 I bought the ground six years ago 

 from a farmer, who had corn in it for 

 several years, but always very poor. 

 I gave it a heavy dressing of horse 

 manure and had it ploughed and sown 

 with Kentucky Bluegrass and Red 

 Clover seed. The grass was a failure 

 and the clover did not come up at all. 

 When I planted the peas I manured 

 very heavily, about 4in. of cow manure 

 and some little horse manure. Those 

 that did grow did very finely, the rest 

 were a failure. It is altogether a puz- 

 zle to me, and I hope you will be able 

 to get at the root of it." 



The diseased specimens this grower 

 sent us revealed some very interesting 

 things. It was plainly seen that the 

 plants suffered from more than one 

 disease. At an exterior view the plants 

 looked healthy, but a closer examina- 

 tion revealed a yellowish, sickly ap- 

 pearance. The seat of the trouble in 

 some cases was confined to the roots 

 only, and in others to that part of the 

 stem just above the roots. In the first 

 instance we found a typical case of eel 

 worms, (Heterodera radicicola) better 

 known as root knot disease in other 

 plants. Those of you who grow vio- 

 lets have surely met with this trouble. 

 It is evident that roots crippled with 

 eel worms cannot make much head- 

 way. Of the other specimens, where 

 the seat of the disease seemed to be lo- 

 cated at the junction between the roots 

 and the stem, the latter was found to 

 be covered with a white weft of fine 

 mycelium threads of a fungus. In 



sijlitting open the stem it was found to 

 be filled with the same white myce- 

 lium and intermixed with small sclero- 

 tia. Cultures of this mycelial growth 

 readily gave a pure culture of a Sclero- 

 tinia, resembling in all appearances 

 the Sclerotinia libertinia. a fungus 

 which causes the damping off of lettuce 

 and so many other plants. In order 

 to estaljlish definitely the relationship 

 of this Sclerotinia fungus to this dis- 

 ease of sweet peas under glass, steril- 

 ized seeds were planted in sterilized 

 pots and soil in the laboratory. All 

 the seeds germinated and the plants 

 were allowed to grow for three weeks, 

 no disease appearin.g on them. Then 

 the pots were divided in two lots; 

 one was left as a check and the other 

 was inoculated with the pure culture of 

 the Sclerotinia by introducing pieces 

 of the fungus in the soil. Both lots 

 check and infected plants, were cov- 

 ered with bell jars to imitate the mois- 

 ture condition of the greenhouse. After 

 four to six days, wilting of the inocu- 

 lated seedlings began, whereas the 

 checks remained healthy. This was re- 

 peated several times with always the 

 same result. This conclusively shows 

 that the fungus Sclerotinia libertinia 

 is able, when present in the soil, to 

 produce a disease on sweet peas under 

 glass. The fungus is usually brought 

 into the greenhouse with the soil, or 

 with the manure. From the account 

 which the grower gave us, it was pure- 

 ly a case of infected soil and this ex- 

 plains why his sweet peas failed. Cross 

 inoculations with the fungus from the 

 sweet pea and the one from lettuce 

 produced typical cases of wilt in each, 

 thus proving conclusively that the 

 fungus from the sweet pea is the same 

 as the Sclerotinia libertinia of the let- 

 tuce, and that the sweet peas are at 

 stake if they are made to follow a crop 

 of lettuce which suffered from this 

 disease. 



Several other fungi were isolated 

 from the diseased stems and roots of 

 the plants above discussed. Inocula- 

 tions with pure cultures of these fungi 

 on healthy seedlings proved to be ac- 

 tive parasites. We are not here giving 

 our final word on these fungi, as more 

 experiments are still uhder way. While 

 working on the diseases of the roots 

 and stems we also investigated the soil 

 which was sent to us by the grower. 

 Accordingly, a few sterilized pots were 

 filled with the soil and sterilized seeds 

 planted in it. and a few other steril- 

 ized pots were filled with sterile soil 

 and also sown with sterilized seeds. 

 The same amount of seeds was planted 

 in both lots of pots. After ten days 

 the seeds in the sterile soil germinated 

 and grew well,, while the seeds in the 

 sick soil germinated but very sparing- 

 ly. Upon closer examination the non- 

 germinating seeds were found to be 

 rotted, and of the few plants which 

 grew one-half had their roots nearly 

 rotted. In culturing these roots the 

 same fungi were obtained. Cultures 

 made from the sick soil produced the 

 same fungi as those found on the dis- 

 eased roots and stems. Our experi- 

 ments are as yet incomplete but the re- 

 sults obtained so far point to the fact 

 that these fungi in the soil are the 

 cause of this soil sickness. The de- 

 bris found in this sick soil also readily 

 yielded a pure culture of the Sclero- 

 tinia libertinia, of which I have just 

 spoken. The fungus diseases just de- 

 scribed can also at any time and un- 



der certain climatic conditions, at- 

 tack the sweet peas grown outdoors. 

 So much for that grower. 



Complaints of another grower of 

 sweet peas under glass reached us. 

 The following are his own words, 

 which speak for themselves: "I grow 

 sweet peas under glass for cut flowers 

 and have been seriously troubled with 

 a disease that spots or mottles the 

 foliage with a yellow color, disfiguring 

 the foliage and flowers, and as it be- 

 comes pronounced a diseased plant 

 never recovers. Both the weak Winter 

 blooming and the stronger Grandiflora 

 and Spencer types are subject alike to 

 it outdoors as well as under glass. We 

 know nothing of this trouble, every 

 grower we have spoken to having a 

 different theory, some believing the 

 puncture of green fly, or aphis, to be 

 the cause. Our observations led us to 

 believe that the use of green manure 

 was responsible. Any light you can 

 turn on this trouble will be greatly 

 welcomed. Early in the Spring of 1911 

 it started in greenhouses and spread 

 rapidly, destroying fully one-third of 

 our crop. Last fall it showed up 

 again. We carefully pulled out all 

 plants showing it and gradually it dis- 

 appeared. As the weather became hot 

 this spring, considerable of it was 

 noticed, but it was too late to do much 

 harm." This closes the quotation. 



As this same spotting, or mottled 

 disease is also a very serious trouble 

 of outdoor sweet peas we shall discuss 

 it later under our next heading. 



Outdoor Diseases of the Sweet Pea. 



Before taking up these diseases it 

 would not be out of place to mention 

 the insects which are parasitic on the 

 sweet pea, although this could consti- 

 tute an address by itself. Among the 

 biting insects we have the "Corn root 

 worm bettle" (Diabrotica longicornis) 

 so called because the larvae often in- 

 jure the corn crop in the South. The 

 beetle is greenish yellow, with six 

 black spots on its back. It resembles 

 the Cucumber striped beetle. Another 

 biting insect is the striped Potato 

 beetle (Epicanta vittata). The cut- 

 worm, when present, often causes 

 great annoyance, sometimes destroy- 

 ing the entire crop. The wireworm 

 also is to be dreaded, especially when 

 sweet peas are planted on a soil which 

 was previously in sod. 



Among the sucking insects we have 

 the red spider (Tetranchus telarius) 

 which is especially troublesome to 

 peas under glass. Finally, the green 

 aphis, which is also very troublesome, 

 is more to be feared in dry weather. 

 So much for the insects. I have men- 

 tioned them not only for the harm 

 they inflict by themselves but, as you 

 will soon see, for the important role 

 they play in spreading about the 

 fungus diseases of the sweet pea. 



As to the fungus diseases proper, it 

 seems that an erroneous belief has 

 crept into the minds of some growers 

 and even into those of some of our 

 sweet pea specialists, namely, that the 

 sweet pea is a plant unusually free 

 from diseases; that the diseases, if 

 ever present, are induced by dry 

 weather or excessive feeding; and that 

 when the plants are growing naturally 

 they are practically immune from the 

 attacks of any diseases. We do not 

 mean to deny that high feeding may 

 predispose the plants to diseases, al- 

 though this has to be experimentally 



